I keep my guns loaded. I don't constantly unload and re-load them. When I drop the slide, I can check the magazine to see if a round chambered.
Unless your gun was plausibly in someone else's possession while you weren't around and maybe they unloaded the gun for whatever reason, why would you suddenly assume your previously loaded gun is now unloaded?
There are many reasons.
Anytime that my weapon is out of my control, I check it. I live in a house with adults only, so I keep several guns loaded and available, but they are out of my control. When I pick one up to holster it, I press check it.
LEO’s oftentimes have to lock their guns up when going into certain buildings. When I used to do that, I would press check when I got it back. I would also do the same if I were a civilian who locked his weapon in his vehicle because he had to go into a building where firearms were not allowed.
When I travel overseas for a mission, we turn our duty weapons into the host country. When I get it back, I press check it.
When I am shooting, every time before I go back to the holster, I press check. I don’t know for sure that the slide picked up a fresh round as it went into battery, and I want to holster a loaded weapon. This training on the range translates into “real life”. I might holster up after a shooting, but I know there’s a possibility that I might need it again, so I want to make sure it’s loaded.
That last paragraph is what I am usually talking about when I talk about press checking. It’s press checking while the weapon is under use.