Most bikes come with threaded holes in the frame where water bottle cages can be mounted. From the factory, bikes have bolts already threaded into these holes.
These bolt threads are dry.
Most bike shops do not grease those bolt threads. If you're lucky, the mechanic will grease them when they mount a water bottle cage you purchased, but otherwise they are left to corrode in place or fall out and leave you with an open hole into the inside of your frame tubes.
These mounts, still sometimes called braze-ons for they way they used to be attached to the frame, also take brackets for mini-pumps and light batteries.
On aluminum and other exotic frames, these mounts are usually Rivnuts. These are pressed into the frame. If you try to break loose a bolt that corroded into one, you could tear the whole thing out of the frame tube. On an aluminum bike, you can probably press in a new one, if the hole isn't too distorted. With carbon, you could have a bigger problem.
Since Rivnut installation doesn't require flame, and torch skills, you will see them on some steel frames as well.
A few minutes and a dab of grease seems like a small price to pay to avoid damage like that.