why typing like this is sometimes okay.

I use it to indicate tone. Proper capitalization and punctuation reads with a formal, cold tone. lowercase without caps reads with a warmer, informal tone there’s a Tom Scott Language Files video documenting it: ▶ this video

I use them a lot to control the tone of what I write. Since periods can indicate a serious or angry tone[1], I often need some other way to indicate a separation between two complete sentences. For example: "It looks like you forgot to do X; that's needed to do Y." Using a period could be perceived as anger, but using a comma would make that sentence a comma splice. [1] ▶ this video

As Gretchen McCulloch put it: the message bubble _is_ the full stop. It did happen organically, and it _is_ an alarming event when someone who usually doesn't use full stops in chat suddenly decides to use one. Tom Scott made a video on that [0], which references Gretchen's book - Because Internet [1] [0] ▶ this video [1] link

Tom Scott has a nice intro video about this: ▶ this video The book he mentions, Because Internet, goes into more detail. But to over-simplify, there were a few major generations of people joining the internet as well as a generation that never fully joined, and each one developed its own conventions for casual writing on their own. Some used the same mechanism to mean different things.