Canadian law requires that a foreign national must be authorized to work in Canada. ‘Work Permit’ is the authorization to work.
Qualified Workers, who have a job offer from a Canadian employer, are admitted into Canada to work in a specific position and location for limited period.
In most case, the employer must apply for and obtain a Labour Market Opinion (LMO) from Service Canada (SC), which determines that there is a shortage of Canadians to fulfill the position, among other factors.
The main advantage is that the person can start working in Canada within a few months of receiving the job offer. The family members of Work Permit holders may also come to Canada. Also, Work Permit holders enjoy many social benefits like free medical coverage, free education for children; can apply for PR status in Canada, gain additional points for Arranged Employment, etc.
More importantly, Canadian work experience and presence in Canada on a valid Work permit help the candidate score additional points under the Federal Skilled worker point system.
Thus, it is easier to convert temporary status into a permanent one, once you are in Canada on a Work permit.
Also, the period spent in Canada on Work permit is counted towards residency requirements for citizenship, once they become PR.
However, recent changes limit the period of stay for a foreign worker to 4 years, after which the individual must return to the home country for 6 years.
Moreover, the above may not be true for people entering Canada for low-skill occupations, which require high school education or on-the-job training/ apprenticeship, only.