Many of us buy watches and are curious to know how old they are or would like to have some idea of their pedigree or provenance. This is not always easy especially for less expensive brands. Needless to say if you have a watch with good documentation, they command better prices at the time of resale.
Serial numbers, photos, detailed descriptions including receipts, guarantee cards and service records all add to the provenance and are essential items some more serious collectors look for when considering buying a timepiece.
If you have a Rolex, Omega or Panerai, this link will help you find out when the watch was manufactured.
Also, I would never buy a watch with serial numbers removed and I always ask for watch serial and model numbers to be included on receipts at the time of purchase. This helps me keep track of what I own or helps provide me with information at the time of sale or for insurance purposes.