For exactly the same reason, its older and more capable brother - JWST - is protected in this way, but much better. His devices are also actively cooled.
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The JWST is thus passively undercooled to -223 °C, but the MIRI device to -267 °C (REALLY cold). SPHEREx "only" to -210 °C. But that's enough for him...
The IR spectrum is of interest to astronomers mainly because it passes through the universe much better. For example, dust clouds are much more penetrable than for visible radiation.
Moreover, as the universe is constantly expanding, it causes that the further an object is from us, the faster it moves away from us. And when it's really far away, billions of light years away, the light that such an object emits is already shifted into the IR spectrum because of that.
This is due to the fact that the light has been on its way to us for a tremendously long time, and in the course of that time the universe has been constantly expanding, and so has space. So, light that originally had a short wavelength in the visible spectrum has shifted into the IR.
This will allow SPHEREx to study even very distant objects. And that's exactly what it wants to do - to accurately map as many galaxies as possible across the universe and find out not only where they are in the sky, but exactly how far away they are from us.
Astronomers are hoping it could tell them information about the period just after the Big Bang. And it lasted a very brief moment. It's the so-called cosmic inflation.
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