I waited a
long time to buy a tablet, because I had a phone, a netbook, a Kindle, and a laptop. I couldn't figure out where it fit into that equation.
But, since getting a Nexus 7 on sale (partly because I wanted to tinker with making mobile apps and wanted to have a tablet sized device to test on) I've found a number of use cases that I didn't realize I had.
The best, most useful, thing is that I use it to work on my Spanish language learning. For the first time in my life, I practice my Spanish every day; for a few minutes before sleep, for a few minutes after waking up, and it replaces my Kindle for restroom visits. I've found my reading comprehension is better than it's ever been, even better than when I spent two years in Spanish class in high school and when I spent more than a month traveling deep in Mexico (where nobody speaks English) after just a short time. Duolingo and Memrise are great tools...so great that in a few more months, when I'm confident in my Spanish, I plan to focus on French. You can use both tools on a laptop, but I don't take my laptop to bed or to the restroom. And, the phone is just a little too small to be effective for this purpose, IMHO.
Likewise, I've found more time for doing online classes...the screen is big enough for videos, but switching between tabs or apps is tedious enough that I don't get distracted the way I do on my laptop or desktop. Again, a use case I discovered after getting the tablet.
I am a bit slow to adopt new stuff like this (but an early adopter of other things; I had a pager for years after everyone else has switched to cell phones, but then got a Sidekick very early in the smart phone story, because there was finally a thing I would want to use a phone for: Browsing the Internet and reading mail), so I dunno what that means for the general case, but I really like my tablet. It gets more use than my phone (because I literally hate phone calls, so my tablet does everything the phone does only better, except fit in my pocket and work without wifi).
"Eventually they end up on the nightstand, and used for reading news and looking at pictures in bed at night, before falling asleep"
Which is a pretty good use case, too. I won't buy a new Kindle when my current one dies (if it ever dies...my first Kindle lasted five+ years before I dropped it on the tile bathroom floor one too many times and cracked the screen), because a tablet works just as well for that use case and has many other uses.