My understanding is that the packs and trim air valves work together in order to achieve in and maintain the desired temperatures of each of the three aircraft zones (flight deck, forward cabin, and aft cabin). Trim air regulation is especially relevant when at fewest one of these three zones is set to a temperature different from those of the others.
The packs will cool all three zones to the temperature of the zone(s) set to the coolest temperature, and the trim air valves will add the amount of hot engine air required for each desired warmer zone to heat up to its target temperature. For example, if zones 1 and 3 are set to 25 °C and zone 2 is set to 20 °C, the packs would cool all three zones to 20 °C, and the trim air valves would increase their temperatures from 20 to 25 °C by allowing the appropriate amount of hot air to enter.
I'm not sure about question 2, as its answer probably varies from airline to airline. Because the effect of trim air regulation seems to be more of a passenger-comfort convenience, I would imagine that, under normal circumstances, the trim air setting on the ramp for some airlines would be as necessary or optional. Trim air should probably be on for engine start, though, as recommended in the FCOM preflight procedures.