For example, if your character is being attacked by archers and you choose to hop behind a tree you should be substantially safer from arrows. If the game fails to reflect that then someone who makes that choice will be frustrated that the game feels wrong because their intuition about what actions are sensible does not match the rules. In a similar way people expect that if their character wields a sword and shield they will be tougher than someone who wields a sword in each hand. It would be possible to build a system where wielding two weapons gave you an option to parry and shields helped you slam people and end up in a situation where putting on a shield actually made you easier to hurt but this would have a bad feel for most people.
This is why systems are so important. People want to do things that are in character and make sense and they also want to 'win' the encounter. If those two goals are achieved by doing the same thing then they feel comfortable and have confidence in the system. On the other hand if being good and making sense are entirely at odds you end up with bitterness between the optimizers and the actors. It is true that if your group matches you very well it hardly matters what system you use because fun will be had. It is also true that if your group is a horrible mismatch for you then it hardly matters what system you use because misery will be had. There are plenty of places in the middle though where a really good system that aligns sensible roleplaying choices with optimized power choices can smooth over the differences between players.
Achieving that balance is one of the cornerstones of Heroes By Trade design. I really want it to be true that if a player just picks a class, picks a few Powers that seem thematically appropriate, and picks a weapon based purely on aesthetics that they will be fine. As good as the twink? Certainly not, but I really want the feel that the best general measure of a character's prowess is their level rather than their ability to avoid making stupid choices that are baked into the game. If you built a character by trying to make a copy of a hero from a movie, for example, that character should at least be reasonable in the system.
The key to doing this is to make sure that any given choice is optimal some of the time. Should I use a greataxe or a twohanded sword? Depends on my stats, my perks, and my Powers. Should I wear chain armour or plate armour? Depends on who I am fighting, how much I have to move around, and where I need to travel to. Should I learn Infested Claws or Whirlwind? Depends on what my teammates are using, whether I am fighting a single target or a mass of enemies, and what other Powers I chose. Any time I find that one option is always correct I tilt things until that isn't true. As long as every choice is optimal given some configuration of the rest of the character things will work out fine.