Lua API has a function lua_getmetatable which will fetch the table with metafunctions if the value has one.
Lua auxiliary library (which is part of lua API) has another function luaL_getmetatable which is a macro that will fetch a value from LUA_REGISTRYINDEX.
But another function from this library luaL_getmetafield with similar name does a completely different thing - it will look for a method in the get_metatable's location.
Why is there two different locations? When is each metatable used?
lua_getmetatable gets the metatable associated with the given object. This is a fundamental feature; if this function didn't exist, there would be no way to access the metatable for a given object.
luaL_getmetatable is part of a convention for giving types to userdata (C objects that can be accessed from Lua) or classes of tables. In this convention you add tables to the registry with luaL_newmetatable, and then use these tables to represent the metatables for different userdata/table types (when you need them you can read them from the registry and set them with luaL_setmetatable).
This is a convenience feature only; and you do not need to follow this convention if you don't want to. Everything will still work if you place the metadata tables somewhere that isn't in the registry and bind them to your userdata with lua_setmetatable. That said, if the luaL_*metatable functions didn't exist, where would you put the tables that you were using to represent the different userdata/table types; and how would you find them again when you needed them for a second time? You could definitely solve this problem in a different way, but why not use the pre-built convention if it works for you.