The bigger question is, why do teams get into these salary cap issues? For these salary dumps to be needed, the teams must already be over the salary cap.
Not the case, TOT. The creation of cap room is about the future.
Realistically, no team would ever spend 100% of the cap. Your cap might be at 100% on paper but for that to occur you need the perfect season. You would have to have your best 22 play every week, win the premiership, and trigger every other bonus for games, best and fairest awards, Brownlow, All-Australian etc......
There's a point in the season where you can adjust a bit but still highly unlikely to ever get to the maximum.
The reason for the cap wriggling is most teams would have about a third of their list as a minimum out of contract at the end of the season and they all want an increase to sign again. Each draftee you have that has finished the set two year deal jumps up by about 200k minimum and more if they are playing regular senior footy so if you don't have players on good money retiring it gets tight quick.
Then you have your rank and file guys coming to their third deal who have become good senior players between years 3 and 5, who need another 200-300k kick to sign them and then if you have a star on your hands you might have to find 4 to 500k to satisfy them.
If you get players who take off quicker than anticipated or get to a higher level than you thought they could then things can get really tight quickly.
If you didn't shift the salary in the trade period, what would happen is when next year comes and you had x amount of guys out of contract you just wouldn't be able to offer them what they wanted and many would leave.