Note, however, that many templates do include the necessary tag to place an article that uses that template into one or more category.
Be gentle, I'm still a bit computer challenged. I need a bit more explanation of this.
When you transclude a template into an article, any categories included in the template will get treated as if they were in the article itself. For example {{wikify}} includes "[[Category:Articles in need of wikification]]" (or something like that), so any article which includes "{{wikify}}" is treated as if it includes "[[Category:Articles in need of wikification]]", so is listed as being in that category.