I think the logic goes something like this:

There are only so many trophy size fish. Historically, people have targeted the bigger fish and left the smaller and weaker fish to breed. Over time, the genetic traits for the smaller weeker fish become more common as the bigger stronger fish genes get 'fished' out of the gene 'pool'.
By changing the mentality of taking home the 'big one' to show off, and being satisfied with a quick picture, and maybe keeping a few smaller fish, we are helping rebuild the genetic traits for larger, stronger fish.
Example - let's say you caught a 40lb steely and killed it. Now you've elimiated a fish that may have breed many generations of potential 40lb fish.
I don't know how scientific this is, but that's how I understand it.

OK, maybe I'm just smoking something here, so if you have any other suggestions as to why this is, please let us know!