I vote for future sprinkler system connection (assuming there is meter at street).
Given exposed exterior supply I guess warm weather climate where sprinkler systems are very popular so "tee" is part of normal install when house was built allowing easy addition of sprinkler system.
Just looked up Rowland Heights, CA. May thru Sep has average high near 90 with less than 0.25" of rain per MONTH. I bet sprinkler systems are quite popular among the green thumb crowd...