There is a lot of writing on how good flashlight is at what it does, but I think it is critical to understand why flashlight is so good in a 'meta' sense to really start to understand Arkham as a game.
As one of my friends put it, "Someone who isn't normally expected to get clues successfully getting 2-3 clues is the difference between winning and losing a scenario in multiplayer"
Arkham, at its core, all about efficiently getting through the act deck, and while there are combat or movement focused acts, the overwhelming majority of scenarios are advanced primarily through investigation: you are, after all, an investigator.
So while a strong, two handed gun, or a deck with every card focused for tackling enemies and threats, is going to be very good at clearing those threats you face out fast, you need to ask yourself why you are removing threats. In general, killing enemies is done specifically to allow you to safely go get more clues. So if your just... not assisting with clues at all, it doesn't matter that you kill enemies twice as fast. Who cares that you killed a 3 health enemy in 1 action instead of 2 because you got rid of your .45 really early in a campaign, if you are going to spend every turn sharpening your sword and polishing your gun rather than using that action to actually help win the scenario?
This doesn't mean every deck needs flashlight: Mystic, Seekers, and arguably Rogues don't really need it usually. Survivors have other tools but often want Flashlight as well. But I see so many guardians falling for the trap option of being 'pure combat' and trying to take that entire dimension of the game on alone, and abandoning all else, and that is a huge mistake.
So Flashlight teaches us this: Your deck needs plans for things to do that actually help your team win the scenario, rather than just endlessly handling threats. Yes, even in 4 player multiplayer. Yes, even if you are Mark.
Killing enemies super efficiently isn't the same as cluvering really efficiently. A seeker, mystic, or rogue just nomming up clues faster than normal is helping the entire team get through the scenario faster, reducing the amoung of encounters everyone faces, and freeing up others to explore other easier locations, regain resources, or explore because they are 'overkilling' a location and gathering faster than normal.
But a fighter like Guardians or Survivors killing the maybe the small handful of 3 health enemies they face early on in a campaign doesn't help anyone else. It just helps that fighter, because no one else really cares how fast the enemy dies, just that it isn't attacking them while they seek out clues. Outside of bosses (who, again, you spend a minority of your time fighting so being a bit less efficient at that early is totally ok), doing lots of damage very fast only helps the team if you can, after efficiently killing something, go on to do other helpful things! So have tools to do other helpful things!
Those tools don't have to be Flashlight, and eventually you may need to leave it behind to get tools to deal with escalating threats, but you need to do more than slap two Evidence! or Look what I found into your deck and calling it a day even once you leave Flashlight behind. Almost every investigator has good options for replacing it, even if it as simple as getting a 'resources for stat boost' card that can boost your investigates.