We all make errors and fatigue (I couldn't sleep much tonight) and speed cause problems but there's also mistakes beyond being careless that indicate in some cases, a lack of education, a deliberate negligence of what is correct, or sometimes pure limitations. For some, it's a forced form of 'voice'.
Well, I use it for a credibility gauge.
If someone is discussing tracking techniques or how to grow something in difficult conditions and his or her grammar is terrible, that doesn't affect (they might write 'effect') that person's credibility in the subject from my perspective. It actually might add to their credibility because of their focus.
If someone is using 'clips' for magazines, carries 'bullets' (as opposed to rounds,) in their AK 'clips', calls that AK "Main Battle Rifle" because they think that term is cool and they don't know there's a specific definition for MBR's and makes other mistakes, they've got little or no credibility in that field...again, unless they're talking about specific experiences they've had with a specific firearm.
If we're discussing the importance of precious metals, it's interesting to see the often dramatic difference in writing skills and cognitive ability between those who understand the historic, social, and economic value of prepping in depth and those who have only the capacity to understand you can't eat gold or silver.