It has to do with range.
Look at it this way, when you zero your rifle at 100 Yards, there is some downward adjustment left in your scope, we refer to this as the bottom of your adjustment.
Any adjustment left in the bottom of your scope is wasted, you will never use it, so by adding Forward cant or angle to the mount you recover some of this adjustment and add it to the top of your adjustment which equals more range.
Originally canted bases or unimounts were used to extend the range of scopes that had a small amount of total adjustment, now most modern scopes have 60 or more MOA.
In the modern scope world it is good to use a canted base to keep the optics as close to center at long range because you will get a clearer cleaner picture when the erector tube in the scope is close to center of the optical path.