Steam is in the details
Latte, with sticker to cover lid hole, Tokyo
I can’t read what the green sticker says, but perhaps it’s to prevent spillage or being burned by hot steam. Maybe it’s for sanitation, to keep the drink sealed until you are ready to drink it?
One use case for a “to go” cup is to take the drink from the counter and consume it immediately. But if you are shopping for others to drink later, or it’s too hot to drink later, how to manage the drink during that transition where it’s in your possession but not being consumed? The sticker lives in that ill-defined period of time.