There are TONS of companies that claim "exclusive" discounts just for signing up for their official spam , both airlines and non-airlines. I don't think it should be counted on a list like this, because it'll so overwhelm the list, and because it's so pervasive (but generally useless to "stack", ie the discounts are usually not all that "exclusive", and can be gotten from lots of diffeent sources).

Also, you said "program", not "airline program". Well, "programs" (unqualified) includes hotel programs and car rental programs. Lots of hotel programs give you free internet if you sign up for their program and then book through their program site (at brands that don't give free internet to everyone), such as Hilton Honors. Lots of car rental programs give you upgrade possibilities if you sign up for their free rental program, such as Avis Preferred.

The obvious unstated "benefit" of most airline programs is the ability to collect and use airlines miles (you can't do that if you don't belong to the airline program at all).