Context

Boo (or an equivalent) is present in nearly all Mario Party since the first one as a board element. Visit her (I'm she/hering my favorite round ghost, I do what I want) and
you can either steal coins from another player, or pay a hefty fee to steal a star.

This is a very strong effect as it's a way to turn excess coins into a 2 stars swing, or giving you money if you can't afford that while denying an opponent money.
It's also the main way for players to catch up to someone front running, because if multiple people steal from them it's going to split up the stars/coins and equalize rankings.

Mario Party 3 introduced an item called the Boo Repellant, which was kept in 4 (as the Gaddlight), 5 (as the Bone Capsule), and 6 (as the Boo-Away Orb) before finally being gone.

When held, if the player is targeted by a Boo steal (be it coins or star), the item is consumed to protect them.
Thus the stealer only spent money (stealing coins cost 5 coins in those games) and in return only consumed the protection.

Another thing to point out is that Boo is usually (at least in those games) hard to reach (locked behind gates and/or out of the way in 3, behind pipes and/or luck junctions in 4, requiring night in 6...).
Because you won't get to visit Boo often, and a star steal is a big swing, that's why I'm considering "wasting a Boo visit" to be a big deal.

Who pops the Gaddlight?

No Gaddlight (base case)

Assuming you are taking the game seriously and playing for first place (the hidden assumption in my Mario Party blog posts), consider the following situation:

You and Mario have 1 star each and in 2~3rd place (depending on coins), while Luigi has 3 stars and is in first with a strong lead on bonus stars.
You and Mario are both arriving at Boo with enough coins to steal a star, and you get to Boo first. What is your play?

We would need to know coins for a fully informed decision, but there is an obvious play we can see: both You and Mario steal a star from Luigi, so you both end up at 2 stars and Luigi at 1.
Luigi still has a bonus star lead but You (and Mario) are both in a good place to contest for first.

Yes Gaddlight (the problem)

Now same situation but Luigi has a Gaddlight. What are your options?

Steal coins from Luigi, using up their Gaddlight. Mario then steals from Luigi and they're both at 2 stars while you're 3rd with 1 star.
Let someone else deal with the Gaddlight and instead steal a star from Mario (we'll see what he does later).
Placing are now Luigi (3 stars), You (2 stars), Mario (0), and 4th player we're assuming doesn't factor in.

If your goal is for you to win, that first play sounds bad because you haven't moved in your ranking, gave Mario a much better chance to win, and also essentially wasted a Boo visit.
That second option becomes a lot more tempting because it gets you closer to Luigi's placement while kicking down your main competition (Mario). So what can Mario do then?

Steal coins from Luigi, using up their Gaddlight. Rankings haven't changed = bad for Mario.
Steal back the star from You, placements are back as they were before except both You and Mario used up coins to secure placings.

From Mario's perspective, that first play would be the worst: they used up their Boo visit and are still 2 stars behind You and 3 behind Luigi.
The second one is the one that gives them a better chance at contesting first place (even if unlikely).

More notes on that example situation

While in here that looks like mutually stealing a star between You and Mario just wastes everyone's coins, keep in mind that:

You and Mario's Boo visits could be several turns apart. Maybe you're not sure Mario will visit Boo on time.
We ignored the 4th player, they could also be on their way to Boo with enough for a star steal.
Mario could lose the coins to afford a star steal before they reach Boo (such as landing on Bowser, a Battle minigame...).
In that case if You steal a star from Mario and they can't steal back it's ideal for You.
If someone pops the Gaddlight, Luigi could also just find another one before someone else gets a Boo visit.

Another note is that if no star steal can be afforded in that whole mess, then the opportunity cost of popping the Gaddlight yourself is lower (since you only miss on coins instead of a star).
But you can do a similar reasoning and find that it's still better if you steal coins and let someone else use their Boo to pop the Gaddlight.

Toxic

Another case of the threat being stronger than the execution. By just having the Gaddlight, Luigi hugely complicated You and Mario's plays and probably shielded him from 2 star steals.