It was useful for asking questions that neither you nor the other person would want to put in a public post. For example, someone posted a picture in the bad real estate sub that looked very familiar, and I sent a DM asking "is this X location?". I might not want people to know I'm familiar with X location, they might not want people to know they're familiar with X location, but maybe we're both ok with each other knowing.
But I'm pretty sure its main use was sending hate to people without getting banned.
Does this count as linework? My understanding of linework is that it's lines that are done in one or two passes, usually with smaller needle counts. Not quite the same as fine line style, but similar? With large blocks like this, you can correct mistakes on the borders by expanding it outward.