The horse stays where you leave it

A saddled horse in vanilla wanders. Dismount to mine a vein or fight something off, and by the time you turn around it has drifted several blocks away - or far enough that you cannot see it at all.

Horse Buff stops that: a horse wearing a saddle holds its position until you mount it again. Unsaddled horses still graze and wander as normal, so wild herds behave the way they always did.