the benefits of joining an alliance are non or very very small advantages for your airline. The main advantage is to have access to the intern forum where you can exchange ideas and strategys, ask for help and communicate with others from your alliance.
Also most alliances share routes between their members 50/50, but also mostly this is only for members which are part from the beginning of the game world.
Feel freet to ask if there some specific questions.
Curse, King Airways (KAW), KLAX