Let me make it clearer. Turn based systems are not just a cosmetic layer on rolls and rules. They are the core combat mechanic on which character features and actions are designed. They are the rules upon which the other rules are built to interact with. In turn based systems, you have an alloted amount of actions you can take before others have to act. In D&D, you usually have a main action, a move action, and a minor action and/or a reaction (depending on editions). You have to plan ahead, because when your turn is over, then everybody acts before you act again. That means if you left yourself in an exposed position, you most likely will get fucked by all your adversaries. So positioning is extremely important, as is the ability to disable others, use cover/conceal yourself, and plan ahead with the rest of your party (who does what and when). You've got all those combat actions/mechanics built around that, such as delaying, readying actions/overwatch, replacing a main action with a move, reactions, etc.