The impact driver is for when you need really high torque. Slightly on the niche side for general DIY. I rarely use mine, as you say, a combi drill is often easier and more appropriate.
To jigsaw safely beside the obvious eye and ear protection:
Make sure your work is held down securely, make sure the blade is in properly and runs in the guide, make sure you keep the cable out of the way so that it cannot fall in the path of the blade and won't snag either.
Take your time, don't push too hard - let the tool do the work. Keep your hand out of the way. You may find you need to keep blowing to get the dust out of the way to see your pencil line.
For a good cut, steer gently, and aim to steer from the back of the saw, if that makes sense. Don't push hard sideways because you'll end up with a very wiggly cut on the underside.
Use the right blade for the job and make sure it's sharp. If your cutting something that has a finished surface like worktop or similar, use a "reverse" cut blade.