When developers generate Steam keys, it allows them to sell their 'game on Steam' elsewhere (their own website, HumbleBundle etc).

This means Steam doesn't take a cut of the sale price. But the developers still get the benefits of Steam community, Steamworks drm, and potentially having a unified update/distribution channel.

Similarly, some gamers prefer to have games 'on Steam' rather than DRM free so that they have a single unified game library.