Here's how I'm thinking about the Kemba trade currently.
Everything starts from the premise that the Celtics will try to land a 3rd player over the next 1-2 years. Beal is the obvious guy who could force his way here, but things always come up. At a certain point, if Tatum and Brown are playing well enough and one of Romeo/Nesmith pops, even consolidating picks/salaries into a Jrue Holiday-type makes sense.
The guy they land in will almost certainly be a perimeter player rather than a big, just because of talent distribution in the league.
Prior to the Kemba deal, if a guy came available, they only really had two choices:
1. Package Smart+TT+filler+assets and still have Kemba post-trade
2. Trade Kemba+assets, in which case you have to include extra assets to move Kemba's contract.
The problem with 1 is that now you have Kemba on a roster with 3 other perimeter alphas, when what you really wanted was more defense+off-ball shooting around them.
In both 1 & 2, you have the problem that, post-draft, your #16 pick, the best draft asset you'll have in awhile, has now turned into a player. Either that player will show a lot right off the bat, in which case he becomes untouchable (think someone like Donovan Mitchell at #13), or, much more likely, you'll need to wait 1-2 years to rebuild his value to that of a raw #16 pick, if that even happens.
If Beal (for example) comes available, it will likely be post-draft, so that #16 was never really going to be one of the assets in play for him.
Now, post-Kemba deal, the trade options look like this:
1. Smart+TT+filler+assets, with Horford and TL at center.
2. Horford+assets, with either TT or fungibig at backup center and Smart on the roster
The nice thing in 1 is that you don't need any further moves to instantly be a contender, although you can make them if needed. This also makes various S&Ts make more sense, for example Lonzo if the Knicks decide not to throw the max and his market dries up a bit for whatever reason.
In 2, you can decide Smart works as a swiss army knife, or move him and/or TT for better fits at the same salary slots.
In addition, getting rid of Kemba's salary gives them room to re-sign Fournier and (if my calculations are correct) still use the remaining $11M Hayward exception on an S&T without going over the apron.