Any ideas for second pipe

Main water supply on left with regulator. 2nd. pipe could have been mistake and/or future line for?
But there is a T with another shut-off. Would a plumber have left the valve behind for no reason?
Any ideas what it could be for?

Sprinklers?

To drain the line.

Gary,
That was a thought, home was built in 2,000 and did not see other homes in the area with sprinklers. Also I questioned a 3/4" line but could have just been following up with the same material to exterior.

Christopher,
Not sure I understand? Just stubbed through wall with no valve on pipe.

… Or exterior hose bibb that froze/burst and was eliminated? 3/4" would be appropriate for either.

Jeffrey,
No signs of damage. Tried the snake camera and the line did a 90* downturn and the lens could not get by.

Possibly a failed water main? Whatever it is, It is abandoned, probably for a good reason.

Used to drain water line.

Auxiliary water line or system like sprinklers or pool.