What is the point of scaling AD runes?

Armor penetration is "better late game" as you said, but for some champs and for cerain people's playstyle, scaling ad will fit their strategies. For example I have 9 scaling ad runes on Zed because I dont need any armor. Riot just gives you the option to choose what runes you take yourself, its entirely up to you whether or not you want to take them. Also Scaling AD runes are cheaper xD

Cheap Easy way to get back up with that extra late game AD if you're behind. Giving you more chances to Make a Recovery.