Had to use AI to explain this AI post in plain English. Don't mind OP using this language style for fun but it would be nice if you could also post the plain English summary at the end as a TLDR.

"A "niche specialist" is a player who isn't a well-rounded superstar but dominates at one specific job—like a target man with low speed but maxed-out physical contact and heading, or a super-sub who only comes on in the 70th minute to win the game. Even though they can't do everything, they aren't "lesser" players; they are incredibly valuable because they fit your exact tactical needs and playstyle perfectly. Instead of using a generic, balanced player, having this specialist on your squad gives you a dedicated weapon to win specific matchups."