I only use a secondary when I want to reuse the yeast cake but the initial beer is not ready to be packaged yet. I’ve added fruit to a primary, although only once. I leave high gravity beers in a primary for months without any noticeable damage. I actually forgot about a braggot in my closet for over a year and it was fine.
It used to be secondaries were recommend to clear a beer too. It will clear up your beer a wee little bit, but from my experience cold crashing gives much better results in clarity than a secondary. Time, yeast selection and overall recipe effect clarity effects that too. Like a lot of folk here, I gave up on the secondary years ago.
To each there own, but for me risk of infection and oxidation is too high for something that delivers minimal results. Plus it’s one less afternoon of sanitizing stuff.