Visibility and self-expression
I guess we all feel the need for people to know that we're not just another face in the crowd.
In southern California (a long way from this car!), where there are endless numbers of cars, self-expression is pushed to new limits. I think they have the highest per-capita number of vanity license plates than anywhere in the world. In the UK, people pay hugely for that kind of self-expression. To have the plate: "5 ROB" costs £25,000, or about $45,000.