Built-in satnavs are (or were) bad.
However they have a roof mounted active antenna (aerial?) which uses the car roof as a ground plane. For two decades this alone meant it received satellites far better than any hand held device inside the car. It is only the newest phones and tables with 20 years improvement in receiver front ends and receiving GPS, Glonass and Galileo (the car only does GPS) satellites that handheld devices have reception to rival my car.
Also the car satnav does fuzzy matching against the map, so if you drive around a roundabout which it thinks is 50 yards further up the road it corrects and puts you on the roundabout. It does this using the ABS brake sensors on the front wheels to detect going round the corners. It also uses the ABS brake sensors for cornering and the speedometer to do dead reckoning when you are in tunnels or lose all satellite reception. No handheld device can possibly do any of that, they don't have the data from the car.
When parked you don't have to dismount anything and hide it in the glove box or take it with you to avoid some scrote breaking into the car to steal it.
But I'll admit a big reason for getting the factory fitted satnav on my 2002 Mini Cooper S is it was the only way to get the speedometer behind the steering wheel next to the rev counter. On test drives of demo Minis I just could not get my head round the speedometer being in the middle of the car. I know it was a retro touch to the original mini, but I feel it was a retro touch too far.
My god. So it wont know about any of the major improvements around Cambridge-Bedford, nor the "missing link" (Swindon to Gloucester) etc etc. Or any of the 20mph/low traffic neighbourhoods/congestion zones/low emission zones. Yes, OK, the major network doesn't change very quickly, but when there is a change it can make substantial differences to journeys and routes.
Indeed it does not, and I often drive on the new A14 north of Cambridge and Cambridge to Bedford roads. It's quite funny watching the fuzzy matching to the map refuse to move because you must be on a road, eventually it gives up and lets you be in the middle of a field.
The car's satnav knows nothing about speed limits, congestion zones or low emission zones. I don't see this as a problem. Despite being a 2002 car it is allowed in the London ULEZ zone and any other one that has the same rules. It's below the official emissions group but the car model is on the exceptions list as it has low enough NOX emissions.
As for speed limits, I use my eyeballs to adhere to those. I don't want a computer telling me I'm about to enter a 20 zone or automatically slowing me down as new cars do.
