The Motability car can be used by or for the benefit of the disabled person, which means that you can elect to have a named driver do the driving for you if you prefer, and this also means you do not have to be in the car for every journey.
Other named drivers in the household can use the Motability car for shopping trips and for other activities, as long as the journey is for the benefit of the disabled person.
Be aware however that only the named drivers listed on your Certificate of Motor Insurance can drive the car and only drivers who have a legally valid driving licence can be added as named drivers.