How much water do you actually drink a day, if I'm being honest? Because a bottle really does help when you're on the go - at work, traveling, out for a walk. You don't have to hunt for a café or store every time, you just have it with you. Plus, if you're watching your pennies, buying water from stores month after month is actually a real expense, even though it seems like nothing. Ecology plays a role too, but I wouldn't get hung up on that - it's more of a bonus.
Now about choosing one. Premium brands like Stanley or Hydro Flask keep temperature better - if you pour hot tea in, it'll stay hot for several hours, and the same goes for cold water the other way around. But you pay a lot for that. Chinese knockoffs (and there are thousands of them on Ozon and AliExpress now) work worse, but for regular water at room temperature you barely notice the difference. The main thing is to check that the lid seals properly so it doesn't leak in your bag, and that the material is food-grade stainless steel or safe plastic.
Here's the catch that people forget to mention: you have to wash these bottles. If you leave water sitting for a few days or drink a milkshake out of it - it starts to smell, can get moldy. The narrow opening is annoying to clean, you need a brush. It's a small thing, but it's irritating when you realize it after you've already bought it. So if you're someone who forgets to rinse out cups, maybe this isn't for you.