What is the point of lua_lock and lua_unlock?

The following implies it's important:

LUA_API void lua_gettable (lua_State *L, int idx) {
  StkId t;
  lua_lock(L);
  t = index2adr(L, idx);
  api_checkvalidindex(L, t);
  luaV_gettable(L, t, L->top - 1, L->top - 1);
  lua_unlock(L);
}


LUA_API void lua_getfield (lua_State *L, int idx, const char *k) {
  StkId t;
  TValue key;
  lua_lock(L);
  t = index2adr(L, idx);
  api_checkvalidindex(L, t);
  setsvalue(L, &key, luaS_new(L, k));
  luaV_gettable(L, t, &key, L->top);
  api_incr_top(L);
  lua_unlock(L);
}

The following implies it does nothing:

#define lua_lock(L)     ((void) 0) 
#define lua_unlock(L)   ((void) 0)

Please enlighten.