The bold tag can be used to make words appear in a bolder font on the web page. Webmasters may use the bold tag to make certain words in their content stand out more, increasing the likelihood of users acknowledging that word whilst scanning the page.
It’s been widely discussed in the search industry that applying such a tag to important keywords will result in the search engines placing slightly greater weight on those words than regular text. Therefore a lot of webmasters tend to apply bold tags to the keywords that they’re looking to rank for, as they think it will increase their search engine positioning for that query. This hasn’t yet been confirmed by Google.
The bold tag can be used for both user experience and SEO, however, as the bold tag hasn’t yet been officially confirmed to be part of the ranking algorithms at Google, I’d suggest to only implement this tag if it’s beneficial for the user.