It usually is because if you were eating a sweet dessert and went to drink non-dessert wine, the wine could taste sour or bitter compared to the dessert you are eating. So, usually dessert wines are sweeter.

there can be up to 150kcals in dessert wine. (3oz glass)

Dessert wine do not need to be aged, but can be. Make sure if it is a white dessert wine, it is chilled.

muscato

There are many places where one could purchase a dessert wine. One could purchase wine a local liquor store or a grocery store like Giant Eagle or Acme.