There is a vast difference between the common tarp-type boat cover and a custom cover that is specifically designed for your particular boat. Typically, the loose fitting boat cover will be used to protect the boat when it is out of the water, during times of inactivity such as winter, when the boat stands idle for longer periods of time.
Why cover it?
The main reason for using any one of the three types of boat covers described below is to protect the boat. The loose fitting boat cover (usually a more generic fit) is fine for protecting the boat when it is stationary, at its place of storage. The problems arise when towing occurs. Many road accidents happen due to the loose cover working its way loose and either blowing completely off or creating uneven wind drag on the trailer and tow vehicle.
Loose covers are suited to keeping the boat free from adverse weather conditions and dirt buildup when the boat is parked or not in use in the dock, but these loose covers can become very dangerous when in contact with strong winds. Remember, towing your boat puts as much wind pressure on the boat and cover as a small hurricane. Simply, if you are driving at 100km/h, your boat cover is experiencing 100km/p winds.