Note: Some gameplay-related spoilers ahead, but nothing too related to the plot/plot twists.
I was almost certain that I wouldn't bother to get NieR: Automata's additional main endings through my own efforts, no thanks to the perception I had that all extra/alternate endings in gaming were the result of tedious replaying with some choice differences here and there. And now that the game's been out for some time, watching those alternate endings - from the five main ones and the numerous joke endings - online is really easy to do. But you shouldn't do that - absolutely not.
Obtaining Ending A and simply watching the rest on YouTube robs those scenes of power and context (the lack of which might inspire you to go back to the game and continue the journey, hopefully, if you had decided on watching instead of playing), and it also means that you're missing out on quite a number of additional gameplay hours that offer new experiences. Endings aside, those new experiences make the extra routes well worth playing through, making the journey as worthwhile as the end (I'm not a fan of the "true" ending however, but I won't go into the why here). Here's what they are.
NieR: Automata's Route B is the most similar to Route A, having you basically replay the same campaign with some slight but noticeable differences, as well as having some sections swapped out for new ones. What really keeps it from feeling too familiar is this - you play not as 2B, but as her Scanner companion 9S.
9S isn't just a different-looking character - his playstyle differs from 2B's as well and fits his android model type. Instead of unleashing heavy attacks, 9S is capable of hacking instead, which results in top down bullet hell sections that differ based on the enemy's strength. The hacking gets pretty familiar after a while, but I didn't find it to be repetitive and grew to enjoy it, although 9S's light attack animatons took longer to grow on me.
This playthrough also results in new revelations regarding some of the enemies you face, notably the opera boss and the inhabitants of the Forest Kingdom. These are welcome backstories, but there are also some scenes that affect the story in bigger ways as well as entirely different sections during the moments when 2B and 9S are separated (and also new side quests). On the less serious side, playing as 9S offers amusing exchanges between the chatty and expressive android and his sterner operator, which also help in further differentiating the experience from that of the first route.
If you've only completed Route A, A2's role in the story might likely have been a source of bewilderment, given how she shows up for only a bit and disappears soon after. For a character featured on the front cover of the game, you'd have expected more, right? Well, that thinking isn't wrong at all, and the truth is there is indeed more of her in the game. Unlike 9S, you'll have to be on Route C to get the chance to play as her, and she's a lot closer to 2B in terms of playstyle. She does differ in a few noticeable ways however, and arguably feels even more lethal than 2B while maintaining the grace and style that pervades NieR: Automata's hack and slash combat.
I can't remember when A2's backstory is revealed (kind of embarassing, since I just completed the game yesterday), but A2 is quite a prominent character in the events of Route C, making her more than just a bonus character. Personality-wise, she's more talkative than 2B but less cheery than 9S, but like the latter she has some amusing conversations with another character.
I enjoyed my time with Routes A and B, but Route C is where the plot gets really compelling. There's also a number of surprising twists to be found here, and if you think that the trailers (particularly Glory to Mankind) gave you a rough idea on what those twists might be about, well, you're probably wrong. Some events aren't surprising by themselves (thanks to the trailers), but the context behind them is actually quite unexpected and far from what the trailers hinted at. Good job at misleading (at least me), trailers!
I really want to talk more about the plot, but since I don't want to spoil the precise details, I'll just say that it carries a stronger sense of focus than the one in Route A/B, partly due to the bigger role character motivations play. There're some new environments present as well. If you still want to take occasional breaks from the story however, there are new side quests to be found in Route C.
This is a bit more of a minor reason than the previous ones, unless you're really into the (plain awesome) combat. A number of weapons are locked away in chests that can only be hacked by 9S, and some of these include combat bracer-type weapons, which aren't as easily obtained in Route A. There's also the matter of upgrading your weapons to level 4 - the maximum level that also grants a bonus passive ability to the upgraded weapon. Playing the other routes gives you more play time to garner the materials you need to do the upgrading, and lets you experience the process and upgrade effects in fresh circumstances.
Another upside is that achieving either Ending C or D rewards you with several things, including a Chapter Select option, which is handy for catching up on unfinished side quests and getting access to the recently released DLC's content without replaying from scratch. There's also a debug room where you can spawn enemies and enjoy the combat long after the credits have rolled several times.
I'm not a huge fan of the true ending, and me unintentionally ruining the first (and simplest) part of the journey towards getting it with my own hands certainly didn't help either. But even so, I definitely see it as something that is worth achieving through playing rather than simply watching online, because like the previous endings, the journey carries weight. I can't say anymore than that though.