Small sites don’t need it
If everything is reachable in two clicks, a search box adds a feature to maintain and a worse experience than the menu. It also invites the failure state of returning nothing for a term you do cover.
Content libraries do
Past roughly a hundred pages, browsing stops working and people expect to search. That threshold arrives quickly for anyone publishing regularly, and it comes with the structure a large library needs.
On a small site, a search box says your navigation didn’t work. On a large one, its absence says the same thing.
Filters often beat a text box
For a library organised by topic, a set of hub links does more than free text, because people rarely know the exact wording. That is what topic hubs are for, and they get crawled as a bonus.
Ranking is the product, again
Returning every match is easy; putting the right one first is the experience. If you build it, the same warnings apply as anywhere — search is consistently underestimated.
Design the empty state
No results is where trust in search dies. Offer the topic hubs, the most-read articles, and a way to contact you rather than a blank page — the same recovery principle as a 404 that offers a route out.
Read the queries
What people search for on your own site is a direct statement of what they expected to find. It is among the cheapest content research available, and it feeds the queue of questions worth answering.