One component of it can be the exchange and transit deals that the ISPs make. "Peering" is generally made under the terms of "sending as much as you receive", so in that way it can be beneficial to limit your outbound, if you are trying to slip into available inbound bandwidth to pair with some business-class outbound-heavy.
But, there are also lots of other business and technology reasons as others go into here.
But, there are also lots of other business and technology reasons as others go into here.