This used to be quite common among small companies which operated a few market day buses in addition to their main business of private hire and excursions. It was uneconomic to buy a separate vehicle for those routes, so they were run with coaches. One typical example was Swanage-based South Dorset Coaches. They mainly did private hire and excursions but also ran two Thursday-only buses, Kimmeridge to Wareham and Creech to Wareham, which were were operated using coach vehicles. Another example was the weekly bus from Bantham to Kingsbridge, in Devon, which was also run with a coach (I can't recall the operator's name).
Sometimes large companies used coaches for longer or limited stop bus services. E.g. Tees used to use coaches for the 93 (Middlesbrough - Scarborough), X4 (Bishop Auckland - Saltburn) and X56 (Middlesbrough - Whitby) bus routes until accessibility needs led to the adoption of low floor buses.