Search engine optimisation isn’t something you should “take a look at” every year or two. Well, you can do it like that, but the results won’t be great. It’s better to be constantly baling out the water than it is to wait until you’re up to your waist before doing anything.
The first reason to be looking at SEO all of the time is obvious: new content. While all material should be written with your readers in mind, it’s also important to consider what the search engines want at the time of writing. Going back and revisiting pages to look at SEO at a later date is both wasteful and much less effective.
Then there are the changes in what search engines want. If you hear of something which might be applicable (such as the new demands for mobile friendliness), the time to take action is straight away. Pencilling it in to discuss at the next annual group marketing meeting is daft.
My third reason is increasing competition. If you don’t make any effort to improve your SEO, you can’t rely on your competitors to be as lazy. And big outfits such as Wikipedia or the social media sites, which are targeting the same search terms, certainly aren’t ignoring SEO in the hope it’ll all work out somehow regardless. Most of those results around yours in Google are slowly getting stronger. What about yours?
There’s more. Your site probably has outdated content on it. This is doing you no good at all, even before Google starts to introduce its rumoured system for ranking pages based on accuracy and truthfulness. Pages which are no longer relevant (or even correct) won’t get links, and divert attention from the current stuff. Get rid of – or update – those pages, although for goodness’ sake, please redirect any deleted pages to their replacements. Fail to do this and “link rot” will set in. You don’t want that.