The bolt at the front is to retension the leather as it stretches and sags over time. lacing on the sides is to control spread when there's a load in the middle. Many do well without any lacing, though if you find the sides flaring and chafing the insides of your thighs, you'll want to use the kacing feature.
Eons ago I used to lace the saddle only through the break-in period, but once the sides take a set I could remove the laces and it wouldn't flare anymore.
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