Why is 'Detect' a legendary action?

Just curious as to how people use a dragon's detect? RAW, its just a perception check - which no player needs an action to perform....so what's the point of it?

Detect. It makes a Wisdom (Perception) check.

In combat, making a Perception check is an action. Inquisitive Rogue gets the ability to do it as a bonus action.

It's a pretty common mistake or house rule to do it differently, but that's beside the point. You asked why, this is why.

This allows them to perform the check out of turn in response to what is happening on the battlefield, which could interfere with the player's plans.

You can read a more in-depth explanation here: https://rpg.stackexchange.com/questions/80394/whats-the-point-of-the-legendary-detect-action

It could depend on how the DM typically treats these types of checks during combat at your table.

thanks!