First let me say I wish ereaders were made by companies who didn't sell ebooks. That, to me, is the single biggest problem with this industry.
But they are and there are some benefits and I thought they were worth mentioning. I can only speak about Amazon as a provider. I have Nooks and Kobos but I don't buy their books. They're for sideloaded books.
The first and most obvious benefit is ease of use. I get 3G Kindles for the most part and I can download books from my library anywhere I happen to be. I can also buy books easily. The Kindles are very well coordinated in terms of features with their books. It all works well.
Also there's the ability to read on my Kindle and later on my phone and then again on my Kindle and never have to find my place. Each device knows where I am on the other device. That also lets me keep a Kindle by my bed so when I go to bed I don't have to carry it with me. I just go to bed, pick up the one on my night stand and read where I left off on my living room Kindle. Maybe I should also get a bathroom Kindle. Actually a front porch Kindle would make sense but it might walk off.
Now I can also buy audiobooks on my Kindle and synch those as well. I haven't done that and I'm not really expecting to but a lot of people probably will so this has to be considered a benefit.
Then there's X-Ray. I used to use that a lot till they "improved" it. Now I just use search to locate characters I've forgotten. It's simpler. But the idea of X-Ray is a very good one.
I can't say for sure none of these benefits would exist if Amazon only sold the books and the readers were made by Asus or Lenovo or Dell. But my guess is they never would have happened. It requires a lot of coordination between reader maker and book seller and there probably wouldn't have been a lot of motivation for that.
I don't really feel locked in. If I bought books from Nook or Kobo I'm sure I could read them on my Kindle just like I read the ones I buy on my Kindle on my Nook and Kobo. It's trivial to do that. But the sellers do make some feeble attempts to lock us in and it seems to be popular to describe this as being locked in.
Barry