I found this interesting article on harbourexchange.com today:

If you’re working towards an RYA or equivalent qualification, then you need to keep a log of all your miles, voyages, night hours, wind speeds , ports visited, boats sailed, trips as a skipper or as crew and days on-board. Even if you’re not working towards a qualification, you’ll want to keep a log so that one day if you do take the plunge to get your Day Skipper or Yacht Master tickets, you have a complete picture of your experience.harbourexchange.com, Sailing CV

You should read the whole article.