The Source SDK Base is the set of required files to play any mods created using the Source SDK.
For example, if you only want to play Source mods, you'll only need the Source SDK Base. But if you also want to develop mods and maps, you'll need both the Source SDK and the Source SDK Base.
Since Valve released the Left 4 Dead Authoring Tools instead of updating the Source SDK for use with Left 4 Dead, there is no "Source SDK Base - Left 4 Dead". Unlike the Orange Box Source SDK, the Left 4 Dead Authoring Tools are only for creating maps for Left 4 Dead, which has all the files required to play the maps (hence the lack of Source SDK Base - Left 4 Dead).