This article is from the Hanggliding and Paragliding FAQ, by Joao Geada with numerous contributions by others.
Many tow operations dispence with an observer prefering to let
the driver perform some of the observer functions. In some cases such
as aero tow, there is no possible way to have a separate observer. In
platform launch, a second person may ride on the platform to try to
deal with emergencies by slacking pressure on the winch or cutting the
rope. In static line tow, the observer may face rearward to operate
the release at the vehicle end of the towline in an emergency. In a
stationary winch tow, the operator faces the pilot and operates the
winch and there is no driver needed.