I think the only good things that would come from not evolving any further than Torracat would be that you can use an Eviolite on it, which may make it bulkier than Incineroar, but that's currently unknown because the base stats of the starters have not been revealed yet. You'd also probably learn certain moves faster if you don't evolve it, but it's probably only faster by a level or two so that isn't very important in my opinion.
If you didn't evolve your Torracat, you'd miss out on the Dark typing and Incineroar's signature move, Darkest Lariat. Incineroar will also probably have higher stats than Torracat, so you'd be missing out on that too. Incineroar will also probably learn more TM moves than Torracat as well.
Hope I helped!