Prior to the establishment of a centralized government run system for probate in January 1858, there existed over 300 church courts where an ancestor’s will could be proved or administration granted.
One of the most important of these church courts, was the Prerogative Court of the Archbishop of York (PCY), second only in importance to the Prerogative Court of the Archbishop of Canterbury (PCC).
Jurisdiction of the PCY applied to persons with bona notabilia (property worth 5 pounds or more) in more than one diocese within the northern Province of York. This included all of the dioceses of York, Carlisle, Chester, Sodor and Man (Isle of Man), Durham and Lancaster, and covered the ancient counties of Cheshire, Cumberland, Durham, Lancashire, Northumberland, Nottinghamshire, Westmoreland, Yorkshire. For these northern residents, only if property was owned in both the province of York and Canterbury, was probate dealt with by the PCC, the senior court. Printed indexes to wills and administrations for the PCY commence from 1389, some 150 years before the start of the first English parish registers in 1538. However, only about 700 (of 11,000) parishes have registers surviving back to 1538. In 1976, it was reported to the House of Lords that since 1831, 2,400 volumes of Parish registers had disappeared.
http://search.findmypast.co.uk/search-united-kingdom-records-in-birth-marriage-death-and-parish-records/and_wills-and-probate">Start exploring wills and probate records
Generally the earlier the parish register, the less information will be recorded. In earlier registers, a baptism may record only the father's name; the recording of a mother’s name wasn't commonplace until the late 17th century.
So when researching these earlier ancestors, wills can have a singular importance, not only because of the detail they frequently contain about family, relationships, property and place of burial, But because of information that may not be recorded or have survived anywhere else. And of course Wills are one of only a handful of documents of a personal nature – apart from diaries – which can give some insight into the inner most feelings of an ancestor.