braveally, my take on this is the following:

Computer applications often need lots of supporting files for them to work properly. If you install an application from a repository, it will make sure that you also get all of the supporting files that the application needs.

It's a bit like buying a computer on its own, or from a computer shop. If you buy a computer on its own it may not have all the accessories it needs to run properly, but if you buy it from a computer shop they will make sure you have all the required accessories before you take it out of the shop.