A torque arms function is to resist the torque or twisting forces generated at the dropouts through the axle. A bicycle dropout, especially front aluminum ones, aren't made with withstand the rather large amount of torque generated by an electric hub motor. The torque arm reinforces the dropouts so the dropout doesn't crack or splay. Aluminum dropouts will likely, rather suddenly, crack or split and the wheel might simply fall off, you'd probably crash in this situation. Steel dropouts might splay or spread in which the axle will turn in the dropouts, likely destroying the wires along with the dropouts.
400w isn't an insane amount of power, but how it effects things is filled with variables. The safe bet is to reinforce the dropouts with torque arms, I certainly would. I'd say if you were using a front kit on aluminum forks to not ride the bike again until you at least installed torque arms, but maybe even consider replacing the forks the steel ones. If you were using something like a steel down hill mountain bike it might be fine to do without the torque arms on a 400w direct drive hub motor, but I'd still probably reinforce dropouts, this becomes especially true if you are using regenerative braking as the axle may end up rocking back and forth, accelerating the rate that things crack or spread.
In the upper right corner there is a search box, just beneath it there is a 'Using google search?" check box, be sure to do searches both with and without this check box checked as the results will be considerably different but both potentially interesting. Before starting threads I recommend always doing 10 minutes minimum of searching on a subject, I promise you your question isn't a new one. A lot of questions come up repeatedly on this forum so I am hoping that by saying this enough the hijacking/derailing as well as redundant threads slows down a bit.