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] https://youtu.be/fS4X1JfX6_Q?t=3m3s
"It looks like you forgot to do X. That's needed to do Y."
I would actually prefer the second one here. It's neutral and to the point.
I guess the problem with the first one is that the reader might interpret the semicolon exactly in the way that you intended it, as an attempt on your behalf to "control the tone". Depending on the personal disposition of the receiver he or she may or may not like it.