[jancsika]

I feel as if I'm being gaslighted.

If the penalty for the specific illegal move of not getting oneself out of check is to lose the game, what could the practical significance of the rule possibly be?

The other respondent is claiming that chess would become a game of "gotcha," as if the rule somehow prevents chess players from faking injuries to distract the chess ref and get free moves.

[tromp]

That penalty applies only in blitz-chess.

In regular chess, the rule just helps a player avoid one particular kind of blunder. Basically, it was arbitrarily decided that game records should never have king captures in them.

I would agree that makes it a poor excuse for a rule.

[jancsika]

I'm so confused-- what is the penalty in normal chess for trying to move a piece when your king would be left in check?

[bmn__]

There is no immediate penalty, normally the opponent notices the illegal move and points it out, and the player has to undo the illegal move and continue with a legal move. Now if the player disobeys, then the opponent is able to declare the game forfeit.

An interesting edge case is if the opponent does not notice the illegal move. See FIDE for details.