Why are some leather saddles laced at the bottom and should I do this to mine? Some Leather saddles, such at the brooks b17 imperial have holes at the bottom for lacing.

What does lacing a leather saddle do and should I (or when should I) do this to my own leather saddle? Should I adjust the saddle tension bolt before considering lacing or do they solve different issues?
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.