Yes, you can use Downy or any other fabric softener sheets in the washer and dryer even if you don't have any clothes inside. Fabric softener sheets are designed to release their softening agents and fragrance when exposed to heat and moisture.
If you want to use fabric softener sheets in the washer, you can simply place a sheet in the washing machine along with your laundry detergent before starting the cycle. The sheet will release its softening properties during the wash cycle, helping to reduce static and make your clothes feel softer.
If you prefer to use fabric softener sheets in the dryer, you can toss a sheet in with your damp laundry before starting the drying cycle. As the dryer heats up, the sheet will release its softening agents and fragrance, helping to reduce static cling and impart a pleasant scent to your clothes.
Even if you don't have any clothes to launder at the moment, you can still use fabric softener sheets in the dryer by placing a sheet inside and running a short, no-heat cycle. This can freshen up the dryer and help eliminate any lingering odors.
However, keep in mind that fabric softener sheets are primarily designed to be used with laundry and may not provide the same benefits when used alone in the washer or dryer.