IELTS is not mandatory for all work permits to Canada. In general, often the unskilled work or labor segment have exemption from language proficiency tests, if you are a graduate, looking for good employment and hoping to get in the skilled segment it is useful to have in hand.
For instance, you are a graduate from India and would be employed in Canada. The person processing your papers, the recruitment office or potential employer does not know your skills in English. Now if you have an IELTS with a decent score, it gives a picture to the prospective employer that yes, this candidate knows how to read, understand, write and communicate in English, so, he/she would not have issues working with the team & clients.
IELTS becomes important if you intend to work under the skilled workers/traders program visa and for applying for PR. What is more important is the route you choose for enter into Canada for work and finally the PR in Canada.
Most potential employees use the Labour market impact assessment-LIMA visa which provides an avenue to work and live in Canada as long as your have a LIMA approved offer of job from the company that you intend to work. So, although not mandatory to certain jobs, it would be useful if you complete it while you are applying for your job.