Why is it necessary to check the pressure of vehicle tires before starting a journey when they are still cold?

Air (whether it is compressed or not) occupies space. This can be proved through an empty syringe experiment, by blocking the outlet and squeezing the pivot as far as it can go.

When the tyre is cold, the air molecules are free to collide with each other as well as the inner walls of the tyre - the tyres thus have a higher pressure

However when the tyre is hot, the molecules are sqaushed together they have little to no space to collide with each other or the walls of the tyre. This lowered movement thus creates a low pressure but a heated tyre.

Hence it is imperative to check your tyres before a long journey to avoid an accident.