1. When some professional creates a niche Q2A website, he already knows what tags will be most usefull, he can predict them by his professional expirience.
For example: Psychologist creates psychological help Q2A website, i bet he already knows that most commonly diagnosed is: mood, personality, eating and other mental disorders. So he can create mental disorders TAGS and describe them (with tag description plugin ) in advance. Such tags can serve as vocabluary for users and boost traffic from search engines. In other language you create more related contet and content is a king :)
2. Sorry, but i can't understand your second question.