The temperature of spaceships can range from -126° C to 149° C. Wouldn't it be better for the spaceship envelope to have as high a thermal conductivity as possible to even out the temperatures (the temperature should come out quite favorably, around 20 degrees C).
4$\begingroup$ spacecraft thermal control can get pretty complicated, but the design goals aren't usually a uniform external temperature. $\endgroup$– Erin AnneFeb 27 at 23:24
First. Keep in mind that temperature is a measurement of the kinetic energy of the particles in the environment. This means that despite the high temperature, the particle density in space is so low that the energy transfer is not really that high.
Second. In space you can only get rid of heat (or gain it) by radiating it, which is the slowest way possible compared to conduction and convection, the main challenge is to make sure it doesn't radiate too much so it doesn't get cold during the cold parts of the orbit, but can radiate enough so it doesn't get too hot during the hot parts. For this spacecraft have radiators installed but their efficiency will vary depending on if they are shaded or "in the sun", so you need to play with that but without compromising the spacecraft operations. (Meaning the spacecraft has to be able to orientate itself to fulfill its primary purpose without having to concern itself about pointing the radiators in the right direction)
Finally. You do have high thermal conductivity in spacecraft to help redistribute heat and redirect it to radiators, but you still need insulation. Let me explain.
Roughly speaking, there are 2 big sources of heat for a spacecraft in the solar system: Solar Radiation and Waste Heat produced as a byproduct of the operations of the spacecraft systems.
Insulation fulfills a double function, keeps the internal heat from escaping when the spacecraft is in a cold environment, and shields the components when the spacecraft is in a hot environment. In the scenario you propose the spacecraft will be dependent on an external factor as temperature (which will change rapidly in space)
When you are in the shaded side of Earth (or maybe far away from the Sun like scientific probes for example), the spacecrafts gets really cold, temperatures drop considerably since the temperature gradient between the vehicle and space is to great and some components might fall out of operational ranges, for others you will even need to have "heat up" procedures in place before you turn them on.
Then when a satellite is in the "sunny" side the opposite might happen, due to the incoming solar radiation, the temperature gradient is reduced and it becomes harder for a spacecraft to get rid of excess heat. Which can increase the whole vehicle temperature to dangerous levels if it's not taken care of.
A LEO satellite normally goes through these cycles several times per day, and needs to be able to maintain ALL its components in an acceptable operational range during that time.
Then you have to consider the payload, propulsion, comms systems. Those will normally operate during specific intervals of time when needed, and will come to add additional excess heat to your vehicle, you need to be able to keep the temperature across all components within acceptable values in the more heat intensive operational modes as well as in stand by mode, independent of where in the orbit you are.
Insulation, paired with proper thermal design, helps you keep control over the overall temperature of your vehicle and grantee it's functioning.
(By the way, there might be a bunch of other reason you need insulation, for example temperature sensitive scientific instruments like in the case of JWST.)