What is broad match, and when is it the wrong choice?
Broad match lets Google show your ad for searches it judges related in meaning to your keyword, not only ones containing it. Google uses signals beyond the keyword itself, including your landing page, the other keywords in the ad group and the user's recent activity. It buys reach at the cost of control, and the only way to know what you bought is the search terms report.
Against exact match, which requires the same meaning as the keyword, and phrase match, which requires the keyword's meaning to be contained in the query, broad is the widest of the three. Google is explicit that it interprets intent rather than text, and that it draws on context from the rest of the ad group and the account. That is a genuine capability, not marketing language, and paired with a strong conversion signal and enough volume it reliably finds converting queries that no hand-built keyword list would have contained.
It fails in a specific and predictable way: it is only as good as the outcome you told it to optimise for. If the conversion action is a low-intent event such as a page view or a form start, broad match will confidently and efficiently find enormous volumes of people who do that thing and never buy. The campaign looks like it is working right up until someone reconciles against actual sales. Broad match does not create that problem, it amplifies whichever problem the conversion setup already had.
The discipline is straightforward and mostly skipped. Read the search terms report on a schedule rather than when something looks wrong, build negatives from what it actually shows rather than from guesses, and check the match type of the negatives themselves, because a negative added at the wrong match type either blocks far more than intended or nothing at all.