Canadian patent law and "filing date" - what exactly is the benefit of having a "priority date"?

According to the Canadian patent info site, here, a patent application can claim a priority date of a previous patent, filed in Canada or in a country with an appropriate treaty, as long as the application as it's no more than a year apart from the priority date.

What's the benefit of having this priority date, especially if there isn't a Canadian patent behind it?

A priority date is an "effective" filing date.

Generally, if two people come up with the same invention, the person who files the patent application first gets the patent.

Consider the following scenario:

1. You file a patent in the U.S. on Oct. 19, 2011.
2. By coincidence, someone else files a patent application on the same idea in Canada on Oct. 20, 2011.
3. You file a Canadian patent application for the same idea on Oct. 18, 2012 -- almost one year later -- and claim priority to your U.S. application.

Canada will give you a priority date of Oct. 19, 2011 so you will have effectively filed before the other person. Assuming all else goes well, you will get the Canadian patent and the other person won't.