That rubber plug covers a fuel pick up tube for the choke. There is another fuel pick up tube for the choke right next to it.
I don't know why one is covered by a rubber plug. And I would be more than curious to know if the carb would still work properly without the plug.
BTW, the plug is about $5.00 plus shipping, depending where you purchase it. I'm thinking a creative mind could find a way to plug that hole without spending $5.
Maybe McMaster-Carr or Small Parts.