Note: I don't mean restaurant staff saying it, but your friends/acquaintances/people you're going out with saying it.
I didn't know this until someone explained it to me, but, for example, if someone invites you to their house for a meal, and they say "eat as much as you want!", it doesn't actually literally mean that in most cases. What they actually meant was "you can eat a lot, but don't be too greedy!".
Also, if someone if giving you a treat, and says you can have anything, they don't actually mean that , and it isn't good to ask for the most expensive thing on the menu. Once, I tried to avoid this by asking for the cheapest thing on the menu. Needless to say, that didn't please my family too much, cos they were sitting next to me.
Only later did I realise by thinking it through, that you can't choose something expensive or eat too much, or they think you are taking advantage of them, and you can't choose something cheap or eat too little, or they might think you are rejecting their hospitality.
Such is etiquette.