You don't NEED it for Eagle. It's mainly to make your life easier. It helps when you are filling out applications. Instead of totaling everything by hand, you can sort by virtually any parameter you want.

It also helps on the line. Instead of keeping a paper log, I use the APDL with logbook pro. At the end of a trip, I sync it with my desktop computer and instantly download the flying I just did.

I have my electronic copy setup to list each entry by day, and I make one entry per month, with all that months totals into my paper log.