By expanding the reach as mentioned previously, they can be good for brand building as well. But relevancy is most important these tags are meant to group together matching contents, videos, and images...even activities. There is no benefit in tagging unrelated content since it will not match with people's search and be considered irrelevant.
When you empower with relevant #tag you are actually grouping your write-up or video with those matching, this is helpful for quick discovery of your post. Since people search using them often they will find your creation as well when they visit a social media site. They also justify keyword variations to explain your topic better. Hence
#interentmarkting #digitalmarketing #onlinemarketing literally point to the same activity but add to keyword variation. It is not known if search engines register them as keywords pertaining to content say an article or blog entry. If SEs do it will be a good SEO or search engine optimization exercise. Anyway, by creating them in an innovative manner you are definitely expanding the reach of your creation. What more do you want as an SEO content writer?
Unique metadata is less visible than the popular ones, but in the case of the latter, there is greater competition. You can create unique innovative metadata to best describe your product or services and they can create a viral impact depending upon the popularity of products and services you are marketing. But using them has become essential, and is considered a best practice. These tags are also used for news, event, and action groupings in a wide variety of manners based on dates, or as general action on events. Eventually, they end up as an exercise to gain popularity of mass following or to create awareness. Nowadays they are being popularly used for activism.
With the gain in popularity, there are many tools available on the Internet for analyzing or tracking the impact of your hashtags.