How to quickly remove all useless iOS applications on my Mac? I have 900 apps on my Mac because I sync both my iPhone and iPad. 

There are a lot of apps that I deleted a long time ago which are useless. I have amassed 40gb of apps.

Is there a quick way to delete all the useless apps?
You could right click on any app in iTunes Apps section, click Show in Finder, then sort by Date Modified and erase all those that haven't been updated in a long time and that you don't use. Be careful because one of your preferred app might not have been updated recently.