Understand how Disk Space is used on your phone. What percentage of your SD Card is filled with music and What percentage is used by Videos and apps Better understand which apps and videos are occupying most space on your phone.Disk Usage app is the best way to begin Disk Cleaner.
Space usage by Apps:
Lets you view apps both in graphical and tabular form, the space used by each app on your phone. You can easily understand which non critical apps are taking space and get rid of them if you desire to clear space.
Space Usage on SD Card:
Figure out what's using up SD Card space using this Disk Digger to find out about Music, Videos, Docs and others taking up SD Card space usage.

Better understand your phone's Disk requirements and before buying a new phone, understand how much memory you need by using Disk Usage with your old phone so that you don't buy more memory than needed.
Figure apps that are taking more space that you expect them to.