We know why an organist needs to work at the piano; the organist typically doubles as the choir rehearsal pianist; the organist needs some piano work to develop sight reading skills, basic musicianship, the hands, and so on; but why should a pianist learn to play the organ? ...
For starters, they can sub for church services and play for weddings, funerals, or graduation and installation ceremonies when a piano may not be available or the occasion calls for the organ to play.
They can widen their association with other musicians and broaden their prospects for part time employment for other organ and piano gigs, and attract piano students this way; if it becomes known that a pianist can play even a little at the organ, over time they'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 we have many more options, plus we get to work our 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, we have more music to play -- 700 years of it.
The pneumatic pipe organ is older than the piano, older than the harpsichord, older than the clavichord ... its sound is older in fact than the sound of any of the instruments we recognize as being members of the grand symphony orchestra today, save perhaps for the drums and harp.
But there's one more reason -- an important one ... maybe the best one of all:
Johann Sebastian Bach didn't have a piano.
If we really want to understand contrapuntal keyboard music, we should try playing it on an organ.