In general, I think the most important aspect of tax treaties that can help give the Canada Revenue Agency the tools to combat tax avoidance and tax evasion is the exchange of information with another jurisdiction. Both this arrangement and the tax treaty contain the current international standard with respect to the exchange of information on request, and therefore it allows the Canada Revenue Agency to request information that's foreseeably relevant to administering Canada's domestic tax laws.
Second, in particular, these treaties contain mini anti-avoidance clauses—in articles 10, 11, and 12, dividends, interest, and royalties—which ensure that someone can't abuse the treaty by trying to access the benefits of the treaty for a reduced withholding tax on any of those particular items of income.
I think those are the two main aspects of a tax treaty that help fight tax avoidance and evasion.