Most of the time floaters are harmless. However, they can be a symptom of a tear in the retina. (The retina is the clear tissue at the back of your eye.) If you notice a sudden increase in floaters or if you see floaters along with flashes of light, this may be a symptom of a retinal tear or detachment.
Roblox is free to play, but to get Builders Club which gives you more features and perks as well as additional Robux you need to use real money to get them.
[edit] Auction system. Advertisers don't buy ad space on ROBLOX. They bid for it in an auction. There is a limited amount of ad space to go around (we only show so many ads per day), so an auction is run and the players who are willing to pay more, get more impressions.