decorum
Writing without proper capitalization, punctuation and spelling is like attending Zoom meetings in your pajamas.
Everybody knows you're there. What difference does it make, one might ask? Why go through the trouble of being presentable when there is no functional difference?
We are not purely functional beings. A significant aspect of human experience is symbolic, not functional. We are drawn to beauty and repelled by its absence. We pay closer attention to people whose voices we find pleasing, and we are more likely to agree with them.
There is an aesthetic aspect of communication that one would be wise never to overlook, regardless of the forum. Make your words beautiful, and they will travel far.