INSTALL THESE APK'S AT YOUR OWN RISK!
It is highly recommended that you install through Google Play.

By side-loading these apps, you risk some issues...
 -Google Play may not detect the installation and thus you will not receive updates or update notifications.
 -Bugs that have since been fixed.
  If installing over an existing installation:
   -Loss of data, especially if crossing between versions that save data differently (check the change logs!).
   -Loss of features, which may prevent access to data or lead to undesired behavior.
   -Other incompatabilities between versions.

Why you might want to side-load:
 -You are unable to use Google Play. Example: You are using an emulator or a different app distribution platform (e.g. Amazon Appstore).
 -You want to roll back to an older version. Example: You don't like the latest version or it's broken.
 -You want to check out what older versions were like. Example: Nostalgia or curiosity.

Again, INSTALL AT YOUR OWN RISK!