For RTS (especially AoE II DE) I use them as:

• the SHIFT key(create 5 units instead of 1 when hold)
• activator for a different action group, where the joystick are mapped to a circular menu. E.g. one of these menu assigns units to one of the 6 quick groups in my circular menu, and I can then select these units with just the right joystick (no button pressed). Another one activates a right joystick circular menu to go to a building (and I did map nearly all building types…)

I can also combine these, to e.g. select all barracks