One of the biggest things I've noticed is that I suck at staying focused. Sometimes I just want to change things up after I've spent awhile in one world with a set of characters. Starting a new book helps me do that! I've been in the mood to try new things all year, so that's making me even more antsy to go from book to book than usual. I like the early stages of a book where you are introduced to the characters and the world. You get a feel for how the story is and how the author writes. I really like exploring books like this and I like to do it often, which can make me wonder to a different book. Even if I'm already loving what I'm reading, I may still start something new so I can be introduced to a different story.
But...doesn't that get confusing?? Starting something new when I'm already reading another books--or three others? Er, sometimes. It depends on the books! If I'm reading multiple books from the same genre featuring similar aspects, then that could get confusing. If I'm reading an adult high fantasy novel, a cute YA contemporary, and a YA thriller, possible confusion isn't a problem.
Speaking of different genres, I also find reading multiple books at once helpful for changes in mood. My mood feels all over the place lately, so I'm not sticking with the same genre for very long. If I'm not in the mood for the contemporary I'm reading, then I can jump over to one of the other books I'm reading. Or start something new! This has a slight downside, though: sometimes reading so much at once makes it tougher to figure out what my mood is. If I'm read two or three books, then I'm fine. Reading much more than that? My mood changes even quicker and I have a hard time getting through anything.
Another thing that has both its positive and negative sides is my tendency to compare books. I can tell which book is working for me more when I'm reading them at the same time. That can be pretty helpful, especially when I go to write about what I didn't like (I tend to forget my minor dislikes when I really like a book). At the same time, though...this can make me like a book less than if I read it by itself. Actually, I can lose some of the connection I had because I take more breaks from the story, and that always makes me more aware of flaws.
Sometimes I lose so much of that connection that I push a book off for weeks and forget about it and have to officially put it "on pause" (similar to DNFing but where I plan to pick it back up soon). It's happened...with quite a few books this year. That's my least favorite part of reading a bunch of books at once! I started The Serpent King in March and I haven't touched it in weeks. It's a decent book, too! Ugh, that poor thing. I've been neglecting it for almost no reason. I like having multiple reads going on at the same time. There's just...some unfortunate side effects. Sadly.
(I WILL finish the books I've put on pause. It'll happen.)