Maintenance is less about keeping a pistol clean, and more about keeping it ready to go when bad things happen, or just for when you want to take it to the range. You may even just be making sure it stays in good shape for your kids when they inherit it. Most guns will outlast the original buyer when properly maintained, but the problem is many people forget this and leave their guns sitting in a safe, drawer, or on top of a shelf in their closet. I've even recovered a few that were sadly stored in a a closet with the water heater. (Just know, moisture + metal objects/tools/guns = rust) Very few guns survive this forever.
I won't get into the merits of regularly shooting a gun you intend to protect yourself with (it is really important DO IT), but even if your gun is going to sit without being shot for long periods of time, it really needs to be checked out regularly. Otherwise you may take it down from the shelf one day and find that a whole side of it is caked with rust. Worse, it may have internal rust you can't even see, but we'll get to that.
For external rust, old style blued guns suffer the most in this. They have less protection against the elements than say those with more modern finishes, like Melonite or Tennifer. Stainless guns, and the chrome and nickel coated guns are also very good, but they too can pick up rust spots, and these can sometimes threaten good function.
Just handling older style blued guns can deposit oils from your skin onto the surface of the firearm. Those oils alone can start to bring on rust given the right conditions. The best rule of thumb, if you take it out, rub it down with Rem-Oil or silicone cloth before you put it away. Much less, if you take these guns out into the field, they will need to be wiped down as well.
Even guns with Melonite or Tennifer can get rust. Especially in areas such as aftermarket sights, as they will not always have as robust a finish on them, and rust can develop where they meet the slide surface.