The flavor profile is different between the ripe and immature fruits.
The immature ones are considerably more sour, but also have more distinct floral/fruity flavors. Odors and flavors like passiflora, papaya, and other things come to mind.
When it is fully ripe it becomes sweeter, more mellow, and less complex seeming.
It can also be sliced ripe and dried out. That form is often used for teas.
Feronia limonia is similar (immature fruits are more sour and have a stronger fruity/floral flavors), both are oddball citrus relatives with hard shells that probably deserve quite a bit more attention.
Supposedly "bael" fruits are often grown at Shiva temples.