Marking gives a clearer order of who should be getting attacked after the first target dies. I'll often use the tank's target as my target, but if they're switching targets to hold hate on each and now the DPS is split, it usually means both enemies live longer and this in turn requires more healing or a chance that an enemy will do another major ability before it dies.

E.g. bees in Sunken Temple, bastards get to do a Final Sting if DPS is split and it becomes stun resistant due to diminishing returns.
Or just killing the little Twintania dragons so they don't spew liquid hell all over the place as long.
In a lot of content, the marking is "silent", e.g. kill order for Garuda sisters, Turn 4 waves, but some form of coordination for single target tends to help the outcome more than being free for all and each party needing to dynamically adapt to another player.