In short: FP makes programs easier to reason about and understand by using trustworthy abstractions. Here are some resources to check out, which explain better than I could:

Why Functional Programming Matters: http://www.cs.utexas.edu/~shmat/courses/cs345/whyfp.pdf

Out of the Tar Pit: http://web.mac.com/ben_moseley/frp/paper-v1_01.pdf

Simple Made Easy: http://www.infoq.com/presentations/Simple-Made-Easy