Back in the day, when handlebars were just wooden sticks to be roughly aimed in the right direction, this category did not exist. Today's motorcycles have a host of accessories and extras that make them precise and easy to control. In addition, sufficient attention is paid to the hands that hold the handlebars and preferably operate and control them as well as possible.
The following product categories can be distinguished:
Handlebar muffs: warm hands without a large investment and without a lot of electronic ingenuity. The basics, but the basics are often good enough. Even when the weather gets extreme.
Heated grips: essential for riders and commuters who brave the elements on a daily basis.
Cruise control: low-budget cruise control, letting the throttle do the work.
Handlebar weights: reduce vibration, but they're also nice to look at.
Grips: because without them, holding a handlebar isn't very comfortable.
Grip covers: sometimes for aesthetic reasons, but just as well for functional reasons (warm hands, clean handles...).
Lever covers: in the same family as the handlebar covers, they are style defining and can be very functional.
Lever guards: on public roads they are primarily a nice visual touch, although they can be absolutely useful in a pinch.
Universal hand protection: these belong in the category "you need your hands, so we'd better protect them".