For most tags, they should be used if they apply to the main plot or main characters. In the case of Crash Landing on You, the Arranged Marriage is between the ML and SFL (two leads), so that is most likely why it is there. While most probably search for it hoping to see it between the first leads, that won't be the case all the time. Some users might think it is important just knowing the fact the ML is engaged to someone else. They may not like that fact, and what to avoid the show, so that tag would be in fact important to them.
It all depends on really for why users search a tag. Would they search it hoping to see the arranged marriage between the main couple, to know if any lead at all if engaged (no matter to whom), to avoid the tags, or whatever else may be the case?