We know why an organist needs to work at the piano, but why should a pianist learn to play the organ? ...
For starters, you can sub for church services and play for weddings, funerals, and graduation ceremonies when a piano is not available.
You can widen your association with other musicians and broaden your prospects for part time employment for other organ and piano gigs, and attract piano students this way; if you can even play a little at the organ, you'll get calls.
Think about it; a piano has one keyboard and makes one sound; an organ has from one to five or more keyboards, plus a keyboard for the feet and can make many different sounds -- thus you have many more options, plus you get to work your feet on a keyboard.
The piano came into use about in the mid-18th century, but we have manuscripts of notated keyboard music dating back to the 15th century, and those manuscripts contain some pieces likely composed in the 14th century and earlier, thus, you have more music to play.
But there's one more overriding reason, and it's this:
Johann Sebastian Bach didn't have a piano.
If you really want to understand contrapuntal keyboard music, try playing it on an organ.