If you have a Will, why does a person’s estate need to go through Probate?

Sorry to hear about your parents.

The purpose of probate is to establish the validity of the Will.  Issues such as whether the Will was properly executed (signed in the presence of the requisite number of witnesses, etc.) and whether the testator (person whose Will this is) had testamentary capacity.  Testamentary capacity means whether the testator had the mental capacity to understand that this is their Will and the provisions of the Will were made without coercion, undue influence, etc.  Probate is proof of a Will.

Depending on the size of an estate, small estates may be exempt from probate.  What is considered a small estate exempt from probate varies from state to state. 

I am so sorry for your loss.  Generally speaking, all estates - with or with out a Will - go through probate in the probate court.  Your Father's estate must have passed to your mother automatically - "by operation of law" - when he passed away because all of their assets were jointly held.  Now your Mother's wishes as listed in her Will have to be carried out.  The only person that can do that is the executor that is named in the Will and the only way that the executor can be appointed by the Probate Court Judge is to submit the Will for probate. The lawyer can not distribute the assets him or herself.  Also, the estate has to pay taxes, debts, etc.  Good luck to you.