The rumour circulating around is that they have the ability to detect which other applications are running or installed on a mobile device. Certainly for applications that have this service installed, they could correlate the owner of the device to an ID. That might help with targeting ads, but it's just a theory.
I think realistically Twitter needed a good crash reporting service for their own applications. They didn't want to build it themselves, so they bought Crashlytics. They get the best crash reporting utility completely tailored to their needs and keeping the service up and running for others is just an added benefit. Maybe they'll be able to monetize it down the road, but for now they get crash reporting on the cheap.
