What are -dbg packages for?
Solution 1:
1) They contain debugging symbols.
2) If you need to debug things locally, then yes, they are necessary to get the debugging symbols. However, -dbg packages are somewhat deprecated, and you should add the ddebs archive to get the dbgsyms packages instead, when possible.
3) Not usually. If you report a crash bug, the crash reporting server will rescan with the appropriate debug packages installed to get a full stack trace with symbols.