Divorce does not have the effect of revoking existing wills (unlike marriage). If you have already made a will, then any reference in that will to your former spouse will be deleted. This can mean that all or part of the will will fail, possibly resulting in an intestacy, or a partial intestacy, so that your estate will not go to the people to whom you want it to go. It is therefore advisable to make a new will after divorce.
Even if you do not already have a will, it is advisable to make one after divorce for all the reasons why it is always advisable to make a will (appointing guardians for minor children, ensuring your estate is left to the people to whom you want it to go, tax planning, and so on).