H.264 (also called AVC, Advanced Video Coding) is the most widely supported video codec in the world - hardware decoders for it are in practically every phone, browser, TV, and set-top box made in the last fifteen years. That universality makes it the safe baseline rung of any OTT codec strategy: if a device can play anything, it can play H.264.
Its weakness is efficiency. Newer codecs (HEVC, AV1) reach the same quality at roughly 30-50% lower bitrate, so serving everything in H.264 means paying more for delivery. The common pattern is to offer H.264 for guaranteed compatibility and layer more efficient codecs on top for devices that support them.
