What are the reasons for crashing a project in project management?

Crashing is a project schedule compression technique in which cost and schedule tradeoffs are analyzed to decrease the project duration with minimal additional cost. A number of alternatives are analyzed, including the assignment of additional resources. Approving overtime pay for project resources is another example of crashing.

For ex: Let us say you create your Project's initial schedule in February and it works out to 12 months. But, your Project Sponsor wants it completed before Christmas this year which means you have only 9 months. You will use Crashing to reduce the overall project schedule to finish the work according to the revised dates

Sometimes, you may realize after a few months into the Project that your work isnt going at the pace as planned and hence you may assign extra resources to bring work back to track. This too will be considered Crashing