These blanks are used when you have a wall plate with keystone jack holes and you don't have enough jacks or keystone inserts to install in all the holes. 

So if you have a 3 port keystone wall plate (a wall plate with 3 square keystone holes) and only two jacks or keystone inserts to install, you can use these blanks to fill the third hole. 

These just simply make the installation look more professional instead of leaving an open hole.