Some sites get some amazing traffic, but aren't able to keep the traffic in the 'off season.'  Well --- as with everything in marketing you have to think about "what are your goals?"; but for the most part we would all like to see more people on our sites...more potential views of our ads, more people learning about your company or  product and hopefully more people converting.  So what are these sites that have some sort of seasonal or event based component to their business to do?  Well --- first off, it is important to win search...so that your site shows up when someone is searching for your solution --- not when you think someone may be searching.  In other words, even though you think the only time most people look for information on swim lessons is in the spring...you better be showing up on search results all year around...because people might want them in the winter indoors and if you can offer that great; if not, have a compelling site that wins search in January convincing a parent that your lessons starting in May are the absolute best.  So really, winning search through blogging is part one.  Part two is engagement.  Someone finds you --- comes to the site once, but why would they come back?  May be they didn't convert the first time on the site and are still in the buying process or aren't looking to buy for a period of time --- however, why would they come back to a site that is simply a brochure style site?  That's the second layer of blogging...people get to know you, value your opinions and have a reason to revisit your site...even when it is -2 degrees outside and you think no one is even thinking about swim lessons.