Is poaching a foie gras torchon really necessary?

Poaching a foie gras torchon is not necessary. Essentially, the liver is a cured charcuterie before the poaching step would occur. From this Serious Eats method of preparation:

To Cook or Not To Cook?

At this stage, the most classical of recipes will have you poach your torchon in a bath of sub-simmering hot water for about 20 minutes, long enough to bring the whole thing into the range of 130 to 140°F, effectively cooking it. More modern recipes, such as Thomas Keller's go for a much, much shorter cooking time—about 90 seconds. Tasted side by side, I've always preferred the shorter cooking time. The foie is denser, has a more buttery texture, and doesn't leak as much fat when you slice it or eat it.

I always wondered why this was until I came to what was a pretty obvious realization: Thomas Keller's 90-second poached torchon is essentially uncooked. I stuck a thermometer into a torchon during its simmer and measured the internal temperature. It started at around 40°F, and finished exactly where it started. Aside from the outer few millimeters, absolutely no cooking occurs in a 90-second poached foie. No wonder the texture is so significantly different—we're essentially eating raw cured liver here!

There is, however, a good reason to poach the foie, even if it's only for a brief period: The exterior layers soften enough that you can wrap the cheesecloth even tighter, giving you a better looking finished product. That Thomas Keller poach is really all about appearances!

For those not familiar, this article from D'Artagnan explains the differences in foie gras terrine and foie gras torchon. (No endorsement intended, just good info.)