The following text is by Adolph E. Peschke as presented in the 1998 printing of the 1993 edition of the Pioneering Merit Badge Pamphlet:
PRUSIK
This knot has the reputation of having a firm, sure grip once it is put under pressure. The multiple opposing turns provide friction and put a bend in the standing part of the rope, which becomes more difficult to pass through the turns as a lateral pull is applied. The prusik is widely used by mountain climbers as they attach a loop (grommet) made from a smaller rope to a larger rope to form a hand or foothold. It can also be used to form hand and shoulder loops for a lateral pull on another rope or to drag a log or spar. Pioneering Uses
- To hook a light tackle on a vertical or horizontal spar.
- To make hand and foot loops for climbing another rope or vertical spar.
- To make hand and shoulder loops as an aid to hauling a large log. It can easily be moved along as the positions require.
- To provide the grip and a loop to tie another line with a sheet bend.
- To provide a safety brake against back-slipping on a load-lifting line. (Do not use when lifting a person.)
SCOUT PIONEERING 
This is another good one, a Prusik is a variation of the Larks Head, which is another variation of the Half Hitch.
I see on the Bushcraft Youtube sites, the Prusik is used along a ridge line to tension a tarp using toggles and a Larks Head. I’ve tried and it works well.