Why teach your dog to sit?
Sit (and stand and down, as well) is a "position exercise" and is an extremely useful skill for every dog to have in their repertoire.
So, why do we teach our dogs to sit, stand, and down on cue?
-alternate behavior (instead of stopping jumping, you can tell him to sit instead)
-teaching polite greeting
-making vet, grooming, and other trips easier
-distracting your dog from scary or stressful situations (sit-ups are great for this)
-keeping your dog's mind busy and active
-prepare for other tasks such as "stay", leash walking, recall, go to your mat
...and a lot more!