Because of it's UNIX background, OSX must keep the correct permissions on system files/folders.
Sometimes installers or applications can change permissions, and this can make other apps 'act up'.
You should repair Disk Permissions at a minimum before you install software updates and after you install them.
It is just good housekeeping for your mac, and it is the first thing a person trying to help you troubleshoot your mac will ask.
Listen to the Podcast here for step by step details.