//So why do we allow impact statements prior to sentencing ?//
Because all victims of crime have a right to make one and, with a very few exceptions, have its contents to be made available to a sentencing court. See section 7 of "The Victims' Code" here:
https://www.gov.uk/government/publications/the-code-of-practice-for-victims-of-crime/code-of-practice-for-victims-of-crime-in-england-and-wales-victims-code
Judges and Magistrates take all manner of factors on board when sentencing. That's why, whenever I comment on a sentencing matter, I make it clear that we (on AB) know only a small fraction of what the court is told. Overarching sentencing guidelines make it clear that the impact on a victim of a particular crime should be considered and the sentence adjusted if appropriate. An example: two cases of burglary. The circumstances of both are similar - a burglar breaks in to a home whilst the single occupant is asleep. But in the first the burglar steals £100 in cash from the lounge whilst the twenty five year old well paid occupant sleeps upstairs. In the second the burglar takes £100 in cash left in the tea caddy whilst the eighty year old widow, dependant solely on her State pension sleeps upstairs. Neither should be burgled but the impact on the second is far greater than that on the first. This is twofold: the sum stolen will probably have a far greater impact on the widow; but also the psychological impact on the 80 year old is likely to be far greater and her sense of security far more severely compromised such that she may not ever be able to get over the ordeal. The 25 year old may simply shrug it off. That's what the court needs to know.
The State operates a deal where victims forgo the right to extract justice on those who commit crimes against them on the understanding that the State does so on their behalf. For that reason victims are an inherent part of the justice process and to ignore the impact of crime on them is to ignore one of the fundamental purposes of the criminal justice system.