I approach it two ways.
Some RPGs are rules with role playing. D&D fits in this camp. It is fun for the same reason any table-top games are fun. The rules create challenge, tension, and fairness.
Others are more role-playing or group story telling with rules. Inspectres comes to mind. Dread, Grim, Thousand and One Nights, etc. In these the focus is on immersing yourself in and/or telling a story. The rules tend to be light are exist mainly to create a shared setting and to provide limitations to help immersion. Immersion requires something not entirely in players' control and some chance for failure, for there to be consequences.
Simply playing pretend can be fun and rewarding. But even in the most unstructured pretend play by young children, you'll see some negotiation of settings and rules and fairness. So perhaps I should throw the question back at you. What is "simply roleplaying" and how does it differ from roleplaying games beyond the formality and complexity of the negotiated parameters used during the roleplay sessions?
Blades in the Dark said:Why We Do This
What’s the point of this shift into a mechanic, anyway? Why not just talk it out?
The main reason is this: when we just talk things out, we tend to build consensus. This is usually a good thing. It helps the group bond, get on the same page, set expectations, all that stuff. But when it comes to action-adventure stories like Blades in the Dark, we don’t want consensus when the characters go into danger. We want to be surprised, or thwarted, or driven to bigger risks, or inspired to create a twist or complication. We want to raise our hands over our heads and ride the roller coaster over the drop.
When the mechanic is triggered, the group first dips into being authors for a moment as they suss out the position, the threats, and the details of the action. Then, author mode switches off and everyone becomes the audience. What will happen next? We hold our breath, lean forward in our seats, and let the dice fall.
1 - to avoid the arguments over who has narrative authority