Some viruses are useful for studying genes - you can make them transfer a genetic sequence into a cell.

They can be used in gene therapy - a very promising field of medicine.

And some viruses could also be used instead of antibiotics, which could potentialy solve the resistence problems and also help avoiding collateral damage to symbiotic bacteria. (Research of this technology is quite old, unfortunately it was abandoned for a long time and everyone focused on antibiotics instead.)