On all but the smallest of sites, you need to have a good search experience teed up and ready to go. This has as much to do with the help it can give you as for the help it can give your site visitors.
Making content easy to find for site visitors has obvious advantages: if they can easily find what they want, they’ll stick around longer. But it also has the advantage of providing you with another source of information on what interested your audience. If they’re searching for it, it’s something they want to know more about.
This can help guide your content creation plans and might help you make changes to your website structure or navigation. (People search for what they’re interested in, of course. They also search for things they can’t find any other way.)
Two caveats: one for very small sites and one for slightly larger sites.
On very basic sites with limited content, search is a bad idea: too many queries will return empty results screens, which is a worse user experience than not having search at all.
On sites with more content but not full libraries of content, you can avoid this problem and still provide useful search by offering filtering instead. Essentially, filtering is a search (or series of searches) in which you’ve defined the search terms already. This can help visitors not have to guess at the language they should use to define their searches and you can still gather the valuable search data through intelligent use of your analytics data.