By maximizing the number of people who find ads relevant to them.
It's easy to miss relevant stuff on Craigslist. I used PadMapper to find my current place. I was the only person that week who put in an offer; had I missed his ad his place would have stayed unoccupied another week at least.
That provides value to the landlord, of course, but since Craigslist charges landlords for access, increased value to landlords means increased revenue potential for Craigslist, which is certainly of value to them.
Craigslist has been quoted, to an audience of investors, with not caring to maximize revenues. Regardless, don't you think Craigslist would see the effects of PadMapper going dark, and that would be the judge of it being a lucrative venture?
The question was how Padmapper provides value to Craigslist. I answered that. The increased value is true whether or not Craigslist wants to maximize revenue.
The most compelling value that Padmapper added to craigslist was that it didn't make me want to tear my own eyes out, like the regular CL site does.