A couple of non Airbus specific answers...
Selected/Managed altitudes and V/S usage
Heads-down button pushing changing your active flight plan is not the best thing to be doing if you are in a busy terminal area and ATC is throwing altitudes and radar headings at you. Chances are you've been given headings that have taken you off your planned track so there's little point in amending the altitude constraints in the flight plan anyway. Much easier to select the new altitude and use open climb/des or V/S.
Good use V/S can be made in terminal areas during the early part of the climb and latter part of descent and also for level changes during cruise.
During cruise, its a passenger comfort thing - if you use say a 500fpm climb, most pax will probably not notice the change.
In terminal areas, V/S has a number of uses.
1) Pax comfort again. To cope with a worst case engine failure at V1 and max TO weight, all twins need to be generously overpowered. Given a low fuel load on a short sector and climb thrust is capable of producing impressive inital rates of climb. While some of us quite like a 6000+fpm climbs, the average pax will feel like they're in an F14 rather than a commercial transport so it's better to use V/S to moderate it.
2) High vertical speeds can panic ATCOs into thinking you're going to bust through your assigned level. So again, use V/S to moderate the climb/descent. eg In the US, ATC requests you climb or descend quickly but then reduce the vertical speed to 500fpm for the last 1000ft. That makes it easier for them to plan movements and they can immediately see if you're going to miss your level while they still have time to intervene.
3) High vertical speeds in busy airspace cause unnecessary TCAS RAs. TCAS doesn't know both you and the intruder aircraft fully intend to level off at your assigned levels - it just sees your combined vertical closure rate as being above its alerting threshold. So again, use V/S to moderate climbs and descents especially with other traffic in the vicinity.
Pack usage
Just drive round in your car for a bit and see the difference in performance and fuel economy with the air-con on and off. Now scale that up to aircraft size! On a hot, high or short runway with a heavy plane, having the packs off or on could be the difference between being able to take off or not.
Normally, the air for engine start is going to come from the APU bleed so it makes sense to switch the packs off rather than have two high demand systems competing for compressed air.