For many companies it is logistically more efficient for staff members to take their vehicles home at the end of their shift. This involves a high level of trust by an employer, as you will never truly know that your vehicle is not being exploited out of hours with non-company related activity.
In addition, a worry for employers is that staff are using the vehicles during the working day for personal reasons, adding miles on to the vehicle and burning through company fuel. When doing this, they are also taking away working hours from your customers.