The .fla configuration allows modders to alter:

Modding GTA San Andreas v2.00: The Ultimate Guide to CLEO and FLA Files

By default, the game engine can only handle a specific number of vehicles, weapon types, map objects, and handling configurations. If you add too many mods, the game crashes instantly due to buffer overflow. FLA files bypass these restrictions, allowing you to add hundreds of custom cars and high-definition maps. Why GTA SA v2.00 Requires Special Attention