Why are airplanes sometimes led by a vehicle?
"Follow-me cars", as they are known, make it easier for pilots to get to the correct spot by the correct route.

This could be the runway before take-off, or the final stand after landing.

They're useful at airports with complex routes and relieve the pilots of some additional navigation burden; their function is one both of safety and convenience.

They might be used as a rule at a particular airport, or on request (if perhaps the pilot is unfamiliar with the taxiways, or there's an urgent need to get to a gate very quickly), or because of local conditions (signage or lighting problems, re-routing because of ground maintenance work).

However, the presence of cars around aircraft and runways is not without its own risks, and follow-me cars in fact add an extra layer of complication to airfield management.

(And sometimes, they have a rather odd marketing role... at Bologna (BLQ) one of the follow-me cars is a Lamborghini. Make of that what you will.)