So what the Engineers do is: switch to the mine kit, throw out their mines, lay out a defensive area, and then pull out their rifle again and start firing. And then, when they are ready, they don't have to switch back to the mine kit to detonate them. They can - if they want to detonate them individually. But they have always the tool belt at hand to detonate all the mines.
All the kits have skills like this. So for instance from the med kit, you actually get your self heal up in the tool belt. And for the grenade kit there is a grenade barrage that comes out of the tool belt. So just because you chose to take the grenade kit, we didn't want you to feel that you always have to be in it to get some of the benefits from it. And that's what the tool belt does.