The work permit allows you to stay in Canada until it's expiry date unless you apply to extend it before it expires. The visa allows you to travel and re-enter Canada. If you won't be travelling outside Canada, you don't need a valid visa. But if you will travel, good idea to apply for one.
Also, note that you can re-enter Canada using your valid permit if you only travelled to the US or St. Pierre and Miquelon island.