Why are some leather saddles laced at the bottom and should I do this to mine?
As a leather saddle wears out, it tends to sink and become softer in the middle while splaying out around the edges. This, naturally, makes the saddle uncomfortable and uneven. Lacing a saddle is a way to revive it and return it to its natural shape, or at least something close. You should lace your saddle if you find that it has become too soft. 

And incidentally, many people punch holes in their leather saddles for this very purpose if the saddle didn't come with them.