We have all sideloaded applications at one point or another in the time we had our Android device, it’s one of the great things about Android. Even downloading from somewhere like the Amazon App Store is considered sideloading because it isn’t the Play Store. When you had updates these applications would show and then when you tried to update them you would get an error.
Because you never actually paid for the application through the play store. Well this issue is now gone because in Play Store version 3.5 and higher these apps will no longer show under your installed app list. Now that list can stay more organized.
Via Droid Life