Saying "check" is not required. In tournaments
Why is saying "check" required in the first place?
Saying "check" is only an unofficial "rule" that casual players erroneously think is a requirement. It is kind of like the "house rule" in monopoly that places on the "free parking" space all of the money paid in fines (luxury tax, chance, community chest). It is a very common "rule" that is not in the official rules.
Saying "check" is to alert the other player that he is in "check".
If he does not know that his King is in check and if he moved another piece, can I take the King?


Saying "check" is not required. In tournaments
Why is saying "check" required in the first place?
Saying "check" is only an unofficial "rule" that casual players erroneously think is a requirement. It is kind of like the "house rule" in monopoly that places on the "free parking" space all of the money paid in fines (luxury tax, chance, community chest). It is a very common "rule" that is not in the official rules.