Do you actually have to wait untill the first of the month? Seems cruel.
Here (in the US) in my state (Massachusetts) one can place the plates (tags) on the new car and drive off! The paperwork is carried out later. So cool!!
The wait would kill me
Actually no, we don't have to wait at all if we don't want to.
But GB registration plates are age-related; you can identify how old a car is by the regular plates and they change twice a year, on 1st March and 1st September. Of course if it's nearly plate-change time then it's sensible to wait for the new one as it's then worth more come resale time (and you have the latest plate to show the Joneses!).
Since the new plates ('57' they will read as opposed to the current '07') are here in under 2 weeks it's sensible to wait.
Even though this doesn't apply if you have 'personal' plates people still wait as when the car is eventually sold it gets regular plates again so it's better to have a later one than an earlier one!