The text explaining what agility does says it affects the different types of armor worn. The perks are singularly unexciting: buffs for running, falling, leaping and tumbling. I would hardly call those buffs necessary. Nice? Yeah, maybe, but certainly not game-changing. As for running, you can get the same effect by putting 3 perks into grit, which not only allows you to run further, but also adds faster stamina recovery plus damage resistance (not to mentions giving you a large stamina pool). Some of the other agility perks even seem more suited for encumbrance or grit.
In most games I’ve played that have an agility buff, agility affects evasion and/or weapon delay. If that were the case in this game, I certainly would consider putting points into it. But as it stands…meh. I’ve never ever put one point into it and I don’t seem to be the worse for wear.