Not quite - SDL is more of an abstraction framework, like DirectX. In the case of OpenGL, while OpenGL itself is portable the details of how you create/destroy a GL window or fullscreen context and get keyboard/mouse events from that window are wholly platform specific. SDL provides a consistent interface for that, and it also provides consistent interfaces for audio, timing, threading/synch, keyboard/mouse/joypad input, file I/O, and CD-ROM access. In other words, you just build your game/emulator and it's pretty much guaranteed to port relatively painlessly.
It's also excellent for people new to graphics/games/emulation because it's generally much easier to learn than DirectX or Xlib or whatever. (The semantics are similar to DirectX given it was originally written to facilitate Loki's Linux ports of Win32 games but not as involved, and there's plenty of sample code around).
