Why is it important to mark targets... from a lvl 50. Yikes

Put simply, it gives your good and bad dps a definitive target. If you lose aggro as a tank and arent marking your targets, blame will be put on you. If your mark your targets and lose aggro from another monster that isnt supposed to be focused to a DPS, then they take blame. It kinda helps you avoid blame tbh.

It really doesnt take much effort on your part to mark (Macro if you have to). Marking also helps BLM/WHM know which targets they should sleep, as well as tell the party kill priority. For example, in Haukke you have the Maidservant. Some dps may not know that they take priority over every single monster in that dungeon, so def mark that.

Also, many players (such as myself) have Macros that, when triggered, apply certain actions to marked targets. If it makes things easier for the team, why not do it?