Before your floor can be treated with a wood sealer- whether as protection or for filling- it needs to be thoroughly sanded or buffed and carefully cleaned to remove any debris, dirt, and dust. A sealer is sometimes also called varnish, although technically a varnish is actually a type of wood floor sealer or, more appropriately, a self-sealing finish.
Yes, some finishes work perfectly without needing a sealer under them. In fact, finishes that cast a film on wood can work well on their own without a sealer underneath. Some good examples are shellac and oil-based finishes like polyurethane and varnish. But lacquer and other water-based finishes need a sealer under them so that they don’t get excessively absorbed by the wood.