You don’t need the expiring deals if you have cap space. The assets returned would be young players / picks.
Yeah, you're missing the point. To trade for a player into cap space you need the cap space, so you need the trade to happen during FA, and you have to dump just about everybody on the roster .
If you trade in season you can send out the expirings, and since we have to let go WAY more money to get max cap space then to match a max in a trade, you keep more assets.
Using Beal as an example:
You could trade for Beal using Smart's expiring, Dunn's expiring and Richardson's expiring.
That takes you into next season with the potential to re-sign TL, Romeo, Pritchard, Nesmith, Grant and the option to bring back Kanter, whoever they sign for the 15th spot, etc.
If you need to clear cap space for a trade you lose Smart, Richardson, have to cut Horford and Grant, renounce Kanter and the 15th spot, etc.
The idea trade is you move Horford in the deal with Smart (or less likely Richardson), allowing you to also bring back the other (or S&T him)