Regularly I read three to four books at a time. Why? I don’t always want to read non-fiction or a biography or fiction or whatever on a given day. On a bad day, some fiction is the way to go. Feeling lost? Biographies often help. Inspiration? Any number of tomes can help that, often in unexpected ways.
If I am in the grip of a compelling read I will set the rest aside and enjoy the journey. But more often than not I like the flexibility multiple reads provide.
Also nice is time to let ideas percolate. Especially true of non-fiction, getting a fresh idea or a reminder of something languishing sometimes needs time to move from frantic acceptance to reasoned integration. Having another read in the mean time helps with that.
And reading an actual bound paper book is a bit of a luxury. Audio books work remarkably well in transit to and from work where the dead tree version or its digital print cousin would not.
I like being able to pick up an eReader or phone or tablet or book, skip back a few pages, and be able to dive back in to a book. Unfortunately when it comes to the digital, when you succumb to one commercial provider you are tied into that provider.