Let me give an example via World of Warcraft. No, this is not going to be dismissive.
One thing WoW has are the talents for each class. These talents fall into three categories: good, bad,, and mandatory. Mandatory talents fall under two categories.
1) Something that makes up an intrinsic part of your class, and is incredibly good
2) Something that might not be incredibly, but the other choices given at the time are very lackluster
You can play without the good ones, though probably not on bleeding edge content and whatnot. However, even simply partying can be difficult without some of the mandatory ones.
The problem with the mandatory feats is that they, too, fall under two categories, but these are much simpler. They are: Cool, and not cool. I played a warlock long ago, so let's look at her. Dark Pact? Cool! It's a pretty damn important part of an affliction warlock, so it's damn mandatory. But it adds something new for me to do and really mixes things up.
But then there's threat.
Do you want to party? You're taking your anti-threat talent. Maybe they've slacked it a bit since I quit, but when I played, it wasn't even a question: You are taking your anti-threat talent. It's boring as hell. It doesn't offer anything cool or noteworthy to you. It doesn't change your playing style. All it does it allow you to play longer.
This is where expertise lies. It's not "I hate only hitting at a ten, I want to hit at a four!" Expertise is "I'd like to contribute in combat. Like, at any point." Expertise is the boring, tiresome, hateful anti-threat talent that you don't want to take because it's just some stupid numbers that don't make the game more fun, but you have to, because it lets you play the game in the first place.