It helps expand their social-graph beyond what you've shared on their platform. It also gives them access to geolocation information for users that haven't shared it previously, as photos shared over text messages may contain geo-location coordinate information. It likely also gives them context about where people are, what they're doing, who they're talking to the most, and what they're communicating about so they can deliver more context-aware advertising.

But if you ask them - they'll say it helps them deliver a better experience for their users and helps connect people, and that's what people really want ... to be more-connected.