Are matches in checked baggage a safety problem? A friend of mine checked in on Singapore Airlines a bag which had a box of matches in it. They are regular matches and not the "strike against anything" type.

My friend subsequently realised that these were banned on that airline, and went back to the baggage check-in location. The baggage check-in staff said that it was not worth the bother to retrieve them, and that security personnel later on would remove the matches if they thought they were a problem.

Presumably there is some chance the personnel later on would miss the matches, but the staff did not seem to bothered by this, which suggested to me that maybe there was no genuine safety problem here.

What's the rationale for this rule existing? Of course in life on the ground matches do not generally spontaneously ignite, but I wondered if maybe different conditions in the cargo hold made matches more likely to ignite on their own?
The reason that matches are banned on most airlines is not that they might accidentally catch fire. Especially not in the cargohold of the plane. Sometimes the cargohold is not even pressurized. and a lot of safety measures are taken to prevent cargo fires, including suppression systems in case this does happen.

Most likely the ban is for carry-on baggage. Most airlines will allow you to take a lighter with you, but only 1. 
The reason is that if someone wants to start a fire in the passenger cabin, this is not as easy with a lighter.
A box of matches on the other hand is a lot easier and faster to set on fire.