Riders often ask me what they should carry with them on the bike in case of a minor mechanical problem.
My own seat bag is a dense mass containing a spare tube, patch kit, chain tool, multi-tool hex key set, individual 8-, 9-, and 10-millimeter box-open wrenches, a spoke wrench and a little scrap of rag. I ride a rural route and sometimes venture onto roads much less traveled. I like to get myself out of a predicament rather than ask anyone for help.
The comprehensive tool kit developed during the time when our shop held weekly mountain bike rides. Many of the participants did not have tools or know how to use them. But I was always the guy with the tools, even 30 years ago when I raced. On the training rides I bothered to carry the stuff other riders just hoped they wouldn't need.
Not all the riders who ask my advice take it. Some of them just carry a cell phone so they can bother a willing supporter to come fetch them back from wherever they broke down. Others want to pick their emergencies. Most often they prepare for a flat tire. They get a tube and a pump or CO2 inflater.
"Should I carry a patch kit?" they ask.
Absolutely. Not only might you patch a minor puncture if you have already used your spare tube, in case you have a serious mechanical problem or -- god forbid -- an injury, you can sniff the glue in the kit to amuse or anesthetize yourself until help arrives.