If you find applications are crashing, not starting properly, or acting strangely, permissions may be the problem. File permissions tell applications what can and can't be done with programs and other files. If a file permission gets changed to the wrong value, it can result in crashes and other strange behavior.
Repairing permissions can often fix these problems.
It's also a good idea to repair permissions before and after performing an OS upgrade.
To repair permissions using CCleaner for Mac: