For example, if one wants to check 'merit giveaway' threads, he could click/search on tags (merit, merit-giveaway, giveaway — could also add ability to sort through boards), imo it will be more convenient to find topics than using search option.
Why should we do it when this forum already has a search engine for you to help you find anything? tags are essentially keywords so you can do searches related to your problem, i think it should only be applied to broader search engines. Forum has been divided into many different sections for different purposes, moreover, the search engine also helps to find the matching keywords. On the contrary, using tags will lead to content confusion, as articles in different sections may use the same tag, making it more difficult to find articles in different sections.