The foundation of a house holds the entire weight of the building. If the foundation falters, the house caves in with disastrous results. A similar concept holds when using a mechanical advantage system for unpinning a boat. That tree trunk with webbing wrapped around it is your anchor on a Z Drag. Both your team and the swamped boat are pulling against that anchor to free the boat. If you wrap webbing several times before adding the pulley, all is well. If you use a simple girth hitch and place the knot under stress, chances are pretty high that it will fail. If you are in the line of fire for that pulley, you will get injured.
An anchor belay is real handy for light weight paddlers setting safety. Climbers use this technique pretty frequently. Stabilization lines often make use of anchors as well. All high lines require anchoring. Good anchors ensure the end of the rope will not move under load and are often used to free people to work on other tasks (like pulling on the other end of the line).