Presumably you answer the scoring questions in other areas.
Simple as that. It's not hard to figure out how a good basketball player helps your team. Jackson is very athletic, he's a versatile defender, he has a very high basketball iq, and if he develops a jumper(unlikely) he's a star.
Versatile wings is a good place to have multiple guys. Jaylen projects as a better scorer and shooter, Jackson should be better at everything else.
A few issues here:
This Boston Celtics team is one that is absolutely filled with "good basketball players". Guys like Jae Crowder, Avery Bradley, Marcus Smart, Kelly Olynyk - these guys are all good basketball players. I'd even go so far as to put Al Horford in that category - though on the upper end of it.
The problem is this...
Good basketball players are players who give you solid production on most nights, and good production on the occasional night.
This is exactly the reason why Boston struggled against teams like Chicago and Washington, and got destroyed by Cleveland. On nights where guys like Bradley, Crowder, Smart and Olynyk played well, we almost always won those games. On nights when those guys couldn't buy a basket, we got blown out.
We relied far too much on Isaiah Thomas' heroics to drag us to victories, and as great a player and warrior as he is, he can't do it all for us every night, not without help. When those "good" players are making their shots, then teams need to respect that and defend those guys, which opens up space for Thomas to do his thing. But when those "good" players aren't making their shots, then opponents sag right off and throw all their attention at Thomas - and no player can face that every night and win.
The difference between a "good" player and a "star" player, is that star players can score more or less at will, and you never really have to worry about them beating themselves - you need to beat them. They aren't going to go 4 games straight without making shots, you need to force them to miss by making life difficult. That requires a lot of defensive attention, which opens things up for other guys.
So as nice as "good" players are, you eventually get to that point where adding more and more "good" players doesn't help.
Al Horford is a nice player, but he's not an alpha dog. He's not a guy who is going to take the defense head on and dominate them all night long and force them to double/triple team him. He's the type who will make the occasional open shots, but more often then not he's going to try to help the team by making plays with his passing. He's not a constant threat to score - teams can lax off him a bit.
Boston doesn't need more Josh Jacksons. Boston needs more Fultzs and Tatums. When the offense isn't flowing, the support guys aren't making shots, Boston needs one or two guys who you can just give the ball to and say "go to work brother". Guys who can break down the defense when nothing else is working and WILL the ball into the basket.
Thomas is the only guy on our team right now who can do that, and if we want to get a step further then we came this year, as a team, we need to get at least one more guy who can take on that role. The Cavs have Lebron and Kyrie, who can both create shots, and even that wasn't enough to beat Golden State. This is what we are facing for the next 2-4 years. This is the enemy we are trying to overcome.
We need more then Jae Crowders, Marcus Smarts and Josh Jacksons to take that next step.