It spreads the fixed costs out over a larger number of students.
The building is there and being used during the day so if you add some classes at night or on weekends the only extra variable costs incurred are salary for an instructor and some minor heat/ac and lighting expenses.
For most of those recreational type classes it only takes about 3 or 4 students paying tuition to break even on cost.
Moore Norman used to have a weekend class that covered building and maintaining garden ponds and water features that was very popular.
Oh Lord ... 
. 
