What is meant by Critical Path and why do we prefer to use it?

The critical path is the sequence of project activities which add up to the longest overall duration. This determines the shortest time possible to complete the project.

by the way ,there can be more than one critical path in a network diagram.

The critical path is preferred to be used as project management tool because: it allows and help the project manager and team to focus their efforts on the most important work packages. Common actions focused on the critical path include: 

• Ensuring that resources are available when needed 
• Borrowing resources from non-critical work packages to ensure completion of the critical items.
• Careful monitoring and reporting for critical activities.
• Smoothing or leveling of resources to most efficiently complete tasks on the critical path.
• Crashing the schedule by adding resources to those items where the most cost effective reduction in duration can be achieved. 
• Identification of work that can be completed simultaneously (fast tracking) to ensure that there are no delays along the critical path. 
• Use of the network diagram to assess the overall risk of the project. If the network has multiple critical paths or one critical path and several near-critical paths, it is said to be sensitive. The more sensitive a project network, the larger the risk of delays and the harder the job of the project manager and team for monitoring task completion.

Salaam alaykum akhi.  This is the fastest path from the start of the project until the completion.  It allows to see what parts of a project must be completed in order to go to the next step.  It allows to figure out for budget and time restraints in order to limit them which ensures the success of a project.

Minimum time needed for project will be derived from Critical path

This is the longest path on the Schedule Network diagram, and is used to define project time baseline. Any delayed activity on this path would delay the project directly, this is why this path is kept in closed focused. This however must be kept in mind that all other activities need to be properly monitored accordingly otherwise there is big probability that the Critical path will change to another path. 

I apologize for the answer I leave the answer specialists experts in the field that's not my area.

Mohammed Helal has an excellent explanation.

thanks for invition ,,,,,,,,,, i agree with experts answers