If your peg holes are pre-drilled, they are good on raising day to temporarily pin a connection. A lot of people use tons of comealongs at raisings, but a lot of times we use the pins instead. They are especially great for braces -- stick the brace in the post and slide a pin in and it has more play in it than it would if you pegged it off hard -- it makes it easier to insert the tenons if you've got a little knee action.
They are invaluable if you scribe for pulling and holding the principle timbers of bents, walls, trusses, etc together while you scribe in secondary members.
If they aren't stainless, don't leave them laying on green oak timbers overnight.
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