Apart from piercing and cutting attacks, the tail needs protection against other threats.
When a wolf person gets pushed or thrown (onto the floor or against a wall), the tail would break if it is in the wrong position (this would be pointing upward, for example). Interestingly, the tail can be moved sideways and survive percussive force from behind (yep, during mating, "doggy-style").
The depicted tail is pretty bushy. The hair could get stuck or, if it's long enough, someone could step on it. A hair net might help to get the "bushiness" under control. And, just like regular hair (on the head), it could be grabbed by an opponent.
So it boils down to tuck it sideways under the body armor.
However, she is a wolf lady. Wolves don't have armored tails, they have armored necks (the hair around their neck is very thick compared to other body regions, to protect against fatal bite attacks). Also, wolves typically fight in packs. They naturally assume defensive positions which prevent attacks from the rear.
Also, wolf vs. wolf fights are usually "ritualized". They fight for position in the pack, not to kill. You may see an alpha wolf almost bite into the throat of another wolf during a fight - just to communicate "I could kill you", not to actually harm his (new) subordinate. Most probably, the wolf girl will be fighting not against other wolf persons but against, dunno, Orcs or whatever evil minions of the Gods. However, there might still be cultural aspects influencing her choice of attire. The tail also facilitates distribution of pheromones (through movements commonly called "wagging"). It may also be socially unacceptable to cover certain body areas, as they used for certain social interactions, like offering one's throat to allow a ritualized attack of the alpha wolf (to show obedience, trust and attachment to the pack). Or other body regions to allow for simulated mating, also for a ritualized way of showing rank and position.