My guess would be this allows them to let the lesser charges go without dropping them entirely or finding him not guilty. I guess they could bring them back into play if he appeals.
That's about it.
The unlicensed and uninsured offences will have been dealt with at sentencing as aggravating features of the main offence of death by careless.
Quite reasonably, you can't be dealt with twice for the same offences.
Had the case gone to trial, a jury may well have found him guilty of all offences.
In that outcome, it's likely the judge would have sentenced for death by careless, and ordered no separate penalty for the other two offences.
The driver only pleaded guilty to death by careless, so the other two offences are still 'live' and must be disposed of.
One might ask why the driver didn't plead guilty to the whole lot.
That's one for the defence barrister, but in general terms the barrister will advise the client to plead guilty to the least number of offences he can to get the overall case dealt with.
Letting the other two lie on the file in this case is 'particularly useful' to the prosecution as a means of finalising the case - the phrase comes from the CPS website advice to prosecutors.
The procedure is irrelevant to any concerned members of public who are trying to assess the overall justice of this case.