hey pooper!!
When our squad is training in how to buff, formation flying, communication, etc, we will seek out an unpopulated base to experiment on - no need for pesky opposition to screw us uo further!! When we want to have fun and a challenge, we hit on a defended base.
So, to summarize, one reason to hit an undefended base is to practise your planning, formation flying, target assessmnet, BDA, etc.