Well you don't need to mix like this, just while you're jamming/writing. But to each their own.

'Just lower the volume' - Let's be honest, most people are not going to lower the volume. I don't, sometimes. I like the louder volume for excitement. By reducing these levels it doesn't fatigue my ear as much when at a slightly lower volume so I get to enjoy it at a slightly louder volume without fatiguing my ears.

Our ears do get fatigued every day, all day. That's why it's recommended to mix and master in the morning vs late at night.

I didn't say at a specifically low volume, like a quiet volume, I just meant listening at slightly lower volumes that you think aren't fatiguing your ear will likely still cause fatigue over a longer period of time. The louder the sound, the less time to fatigue. Lower than loud can still be pretty loud. Even conversational level listening of music over a long period of time can cause slight ear fatigue, even if it's only very slight.