Its the easiest, cheapest way to get the attention of others without actually having anything pertinent to say. Easy misspellings would be second.
This is only a means of communication. We (BIG generalization) know the language and how to use and abuse it. Typing a draft for a RFC is somewhat different; I have my whole team looking over my shoulder and nitpicking every comma and word usage. Here, we, I, anyway, post in first draft mode as we would speak. As in meatspace, I often think about what I am about to say before spewing, but sometimes I don't. At one end of the posting spectrum, we have people who post in a convoluted style which forces one to make an attempt to understand what is being said. At the other, we have posters who attempt to post in the style of a southern gentleman, parodying a migrant worker quoting a drunken buffalo. In Alaska.
If it makes a difference to the content (uncle jack and the horse, for example), it's not unreasonable to check on the spelling and punctionation. If it goes though without ambiguity, however, the message is still understood. If all you have to do is complain about the syntax, punctuation, and font type, you have way too much time on your hands. If you have something to say, just fucking say it. If you have doubts about whether your sense of humor is understood, or even doubts if you actually have a sense of humor, there are a plethora of emoticons (disgusting little buggers...) that you can use.
We're just here to talk, not bitch about the language. Or write a new and improved (I hate that phrase) dictionary. No, not dictionary.
Picees... place?? oh fuckit, Amy, you do it...
Hugh