When you get on a plane you hope that it has been checked over before take-off.
Forklifts and scissor lifts must be inspected before use as well. This involves a visual check and then an operational check. It doesn’t need to take long but it does need to be done and documented. The best way to ensure this becomes a regular and meaningful activity is to keep a binder of checklists on or near the machine. Operators are responsible for doing this and supervisors are responsible for making sure it is done.
Why do it? Same reason as the pre-flight check on the plane, to make sure everything is operating correctly and that the forklift is safe to use.
For more useful forklift safety resources: http://www.martinsforklift.com/forklift%20safety.htm