As I say to my clients: start with the desired outcome and work backwards from there. What is the desired outcome from using, in this case, tags? From what I've seen on related questions, tagging g2g posts help volunteers find questions they are most likely to be able to answer (e.g., England, Germany, Poland; colonial, civil_war). A related desired outcome is that people who ask questions attract those most able to answer them.
What other desired outcomes are sought by the use of tags?