I think it feels much more natural to read ebooks and pdf lecture notes on a tablet than on a laptop. I like watching films or playing basic games on journeys as well, but don't really want to take out my whole laptop in a small car a busy train. I don't actually have a tablet myself, so I currently use my phone or laptop for these purposes - but I do often find the phone screen a bit too small, or the laptop not be quite portable enough.
A tablet is also good when you just want to do a small task quickly, then go back to whatever you were doing before. Switching on your laptop and waiting for it to boot up seems a bit of a hassle if you just want to watch a short YouTube video, or do a quick Google search, or read one news article. Plus, most of the time you leave your laptop plugged in upstairs on a desk, whereas it's easier to carry a tablet around the house with you, and conveniently find that it happens to be nearby when you need it. Especially if you want other people to be able to see the screen as well, e.g. a few people flipping through a photo album together, or playing a turn-based multiplayer game, it's better to have a bigger screen on a tablet, than trying to do that on a phone.
I suppose since there isn't much you can do on a tablet that you can't do on a phone or laptop, it's debatable as to whether a tablet is actually worth all the money that it costs. But it certainly can be much more convenient at times, to use a tablet.