The APOe4 (https://www.snpedia.com/index.php/APOE) gene's connection to Alzheimer's is one of the most well-studied. Roughly, if I recall correctly, one allele doubles your risk and both multiplies it even further. There's some evidence (probably weak, but still) that exercise limits that risk https://well.blogs.nytimes.com/2014/07/02/can-exercise-reduc...

Now, you could say "everyone should exercise", and maybe so. But any tangible evidence for the long-term benefits helps. Sure, there's general evidence of benefits, but in my (limited, dated) exposure to the literature, general exercise research is not necessarily compelling enough, especially given the difficulties in pulling out correlations between exercisers and other associated activities or traits.

I find exercise boring, and I'm definitely looking to minimize exercise necessary to maximize its benefits.

By the way, the genetic research on predisposition to side effects to drugs and such is also extremely compelling. It's sad that the medical world is still so far from even trying to do precision medicine.