Plant it to grow Cave Background. Technically, this is also Dirt, it's just dirt that's farther away.
Cave Background Seed price in Growtopia: sells for about 67/WL, buyers pay about 100/WL (median of 8 offers from community Discords, past 30 days). Live price chart and the latest buy and sell offers.
Recently sold at: ODCS, COCSIS, XBMARKET, 4R46, PETAWNI