Also;
The APU is a gas turbine engine running on high internal temperatures, so the longer you can leave it to thermally stabilise before making big power demands, the better for its health and longevity.
You wouldn't select TOGA straight after starting a main engine from cold, or if its oil was cold, and the same general principle applies to the APU.
Starting the APU then immediately selecting APU bleed and one or both packs puts a hell of a load and strain on the internal components. When I was PF setting up the cockpit I would start the APU, then do some other tasks, then a few minutes later, switch on the APU bleed, then continue setting up the FMGS and another few minutes after that, select a pack on - if circumstances allowed.
You can use the APU straight away in an emergency, but if you don't need to, it's better not to. You might need it in a real airborne emergency one day, so best to treat it gently as much as you can.