Tim, there are many possible points:
- Not having to pay TomTom/Garmin/whoever 100EUR each year for an updated map.
- Having the map updated more often than once each year.
- TomTom doesn't currently offer a map of Greece. OSM could (in the future).
- None of the major nav system vendors are likely to be offering maps of Congo or Laos any time soon (to pick two examples at random). OSM could.
- Providing free maps to Wikipedia, books, whatever.
- Non-car users - there are bunch of people at OSM concentrating on the UK's national cycle network, for example. So you'd have the prospect of one day having a specific routing device (or at least a specific map) for cyclists.
- Providing richer maps. Marking each post box, telephone box, whatever people are interested in finding.
There's not enough money in several of these points for TomTom or Garmin (or Navteq or whoever) to be that interested in satisfying them.
Most importantly, it's fun to do.