Many possibilities:

- gain the remainder of users who perhaps can afford a bike ride but not an uber to their destination

- gather bike-specific data which could immensely improve safety for autonomous vehicles

- improve their horrible image by aligning themselves with environmentally-friendly and traffic-alleviating transit

- bikes are fully automated and require no troublesome humans to keep the system going. Uber drivers a big hassle for Uber.

- optimizing a bike rental system is a training ground for running an autonomous car rental system

Just off the top of my head..