Why should I put a coin on engraving

Solution 1:

Generally speaking, for an Elbereth square to be active (that is, for it to actively ward off monsters), something must occupy that square. In many cases, this will simply be you standing on it, but if you want to have an Elbereth square maintain its potency while you're not standing on it -- say, to protect your stash, for example -- you need to drop some sort of item onto the engraved square to keep its warding properties activated. Dropping a single gold piece is a cheap and convenient way of doing this, as it's a low-value item, you're unlikely to confuse it for something else, and you're generally always carrying some on hand.

I'm not aware of any reason why you'd want to drop a coin onto non-Elbereth engravings, however, as standard engravings don't inherently do anything.

Solution 2:

This only applies to Elbereth engravings. They're the only engraving with a genuine in-game effect.

An Elbereth engraving only has effect if you're standing on it or if there is at least one item on it. The idea is that it protects you or your stash.

If you're using Elbereth to block a passageway, you need to make it look like a stash by dropping at least one item on it. A gold piece is an item that you would normally carry and that you can spare. Better: drop an item that even Elbereth-ignoring monsters (@, mostly) won't pick up. A cursed useless ring is a good one. If you have a pet, the item should be cursed to prevent it from picking it up; this will make it hard for your pet to follow you across that square if needed.