The whole point is to ensure that your conversion is safe and legal. Cars have to be built to certain specifications from factory, and you also need to meet these requirements if modifying the vehicle.
A lot of them go 'by the by', particularly emissions/noise, but the big ones like competent installation work and suitable safety equipment (eg. you may not be able to get a certificate without a brake upgrade, depends on modifications, car, and the engineer you consult) are an essential.
This is what the certificate will cover. Driving your car without it is no doubt illegal and will also leave you uninsured (eg I did a 1GGTE conversion on a car, when getting a mod plate one engineer refused it without a brake upgrade and another gave the mod plate with no worries).