You can be more varied of character rather than limited to the capabilities of a certain class as long as it makes sense why they'd have their skill set and not be overpowered by being great at everything. Due to not being great at everything, people naturally fall into specializations in time to become a more efficient team rather than everyone shooting arrows and then some fumbling for their shields to become the wall, while others try to start using magic that they only learn part time to allow for other skills, etc. People who try to do every job equally would likely do bad at every job.
The gm can roll if they can't decide the outcome or want to leave an element of chance/surprise to keep things interesting, given that our written descriptions of our actions doesn't guarantee a certain outcome. The gm might think, "well what if the dragon does happen to arouse from his slumber while he's out there?" It's up to the gm's discretion. There's no stats to add to dice rolls, but rather just probability of possible outcomes based on logic from our written descriptions.