SDL provides its own implementation of some of the most important C runtime functions.
Using these functions allows an app to have access to common C functionality without depending on a specific C runtime (or a C runtime at all). More importantly, the SDL implementations work identically across platforms, so apps can avoid surprises like snprintf() behaving differently between Windows and Linux builds, or itoa() only existing on some platforms.
For many of the most common functions, like SDL_memcpy, SDL might just call through to the usual C runtime behind the scenes, if it makes sense to do so (if it's faster and always available/reliable on a given platform), reducing library size and offering the most optimized option.
SDL also offers other C-runtime-adjacent functionality in this header that either isn't, strictly speaking, part of any C runtime standards, like SDL_crc32() and SDL_reinterpret_cast, etc. It also offers a few better options, like SDL_strlcpy(), which functions as a safer form of strcpy().