In most counties in the United States, probate records come in one of two forms: record copies of legal documents created during the probate process which are typically recorded in bound volumes and files/packets containing original copies of documents that were used to settle up the estate.
Depending upon the time period and the location, there may be several different types of record volumes of probate records: bonds, wills, appraisements, inventories, etc. Occasionally the record copy contains handwriting that is easier to read than the original document or annotations that are not on the original. The file/packet of loose papers may contain items that are not recorded in the bound volume.
For those reasons, it is advised to search both the record copies and the file/packet of papers–if the location has them both.