You may one day experience a problem with your mac and then need to get the music from your iPhone back to your computer.
There are 3 solutions to this problem
You should have used iTunes Match
Then all your music , including ripped music is stored with Apple online
More information can be found here
Solution 2 & 3 are software solutions designed to get the music from your iOS device ( iPhone / iPad ) and back to your computer so you can reimport into iTunes