They're basically vehicles now, as far as I can tell. You can get in and out of them, but they'll just stand where you left them, you can't carry them around in your pocket anymore. I'm not sure how necessary they really are and if it's possible to go through the game without using them.
I've heard that if you leave your Power Armor somewhere with an energy cell in it, an NPC might actually steal it. Which is why I have two of these things already--I stole one from some raiders, then killed them all with it.
This Fallout has a lot more very high-powered enemies than I remember encountering in Fallout 3 or New Vegas. I had to complete a mission up by Salem which was flooded with Legendary-level Mirelurks who also had the little "you don't want to do this" skull next to their names. I was way underpowered, and spent most of the time running from objective to objective and toying with the limited movement of the Mirelurks.
I'm not sure how necessary they are, but for the intro mission for the Brotherhood of Steel, I donned my Raider Armor and joined up in that regard. I don't think it was necessary, but it certainly helped.