This is a good way to ensure that you only have one war at a time -- make sure that at least one of your frontiers is well-enough defended that whoever borders on you there will be unable to break through.

Now, it's true, no border can ever be inviolate. But, if your forces there are large enough, your enemy will need to build up for a turn or two before they can attack, and that will at least give you a chance to prepare for war.

And a strong army can be a large enough deterrent to put off that inevitable war a long time -- giving you a chance to grow and confront that conflict from a position of strength.