Reading, Study, Word/Topic Study, occasionally Notes, more frequently Copy and Paste projects for various purposes. An example of a copy and paste project is to collect sections from various resources on the same topic. I also do things like copy and paste public domain works I've purchased from Logos, and create modules for the free TheWord Bible software and in the process make them better than the Logos version.
The greatest advantage of digital books is they do not age with time. Take a brand new paper book and inspect the paper color and quality, and then observe over time even as few as five or ten or fifteen years many if not most books (which are not printed on acid-free paper) will begin to show age the paper will turn yellowish (decreasing ease of reading) and depending on the paper quality start browning eventually becoming brittle and falling apart. Another great advantage is space and weight, paper books can take up great amounts of space (I have too many myself), and moving them around can be quite a chore, even within the same house. Another advantage is the ability to enlarge digital texts (which tend to be sharper along with bright background), another is they do not get damaged or wear out, and it's easy to highlight and unhighlight digital texts with no bleeding or worry about defacing and therefore decreasing value for collecting or re-selling, etc.