Building solely offense benefits your team, because poke can create pressure which either leads to kills or forcing enemies to flee. That is indeed how poke supports work, and even every champion in the game who isn't a tank or utility/CC champion. Not every kind of support has to spam Ardent Censor, Redemption, and Locket. Pressure is just as valuable as CC or utility with a good team comp. And claiming that building pure damage doesn't benefit the team is stupid because that's what assassins, bruisers, mages, and ADCs do.
Additionally, if you want to discuss items that an AD support could possibility build, I have a list for you:
Frozen Mallet
Black Cleaver
Blade of the Ruined King
Mortal Reminder
Death's Dance
Duskblade of Draktharr
Rapid Firecannon
Maw of Malortius
Wit's End
Runaan's Hurricane
Edge of Night
Trinity Force
Statikk Shiv
Yomuu's Ghostblade
Ravenous Hydra
Titanic Hydra
Every single one of these items can easily be built on a support budget. The most expensive single component is a Pickaxe, at 875g, and the rest are composed from Daggers, Short Swords, Brawler Gloves, and Rubies. So don't tell me that an AD support can't afford items when there are clearly plenty of items that they could build to scale up their AD.