See that chip on the end of the key? That’s the transponder that the immobiliser needs to recognise before it will allow the car to start. If it isn’t the right chip already programmed into the car, then it won’t start. This particular chip is a VW ID48, sat on a HU66 key. It’s just been programmed into to a VW Transporter T5.
Modern vehicles (since about 2000) are fitted with immobilisers to prevent theft. The immobiliser checks for the correct, matched chip before it will allow the car to start. Some vehicles will just not turn over if the chip isn’t recognised. On other vehicles the engine will turn over but will not start due to the fuel pump being disabled.
Some recent cars will start without putting the key in the ignition. The range of these can be many metres. This is the reason you shouldn’t keep these keys in the house near the car. Even if the signal doesn’t reach the car, it can often be copied and relayed. You can buy special pouches to protect this type of key. Or at least put them in a metal box of some form at night at the other end of the house.