One way to think about the semicolon is in terms of pauses.
When you are reading and you hit a comma, you pause briefly, and then you carry on until you hit a full stop. This makes you pause a bit longer, perhaps even take a breath. And then you carry on again.
- So if a comma is a small pause...
- And a period or full stop is a long pause...
- Then a semicolon is sort of medium pause: less than a full stop, but more than a comma.
Why would you want this medium pause? Two possible reasons.
First, you want two statements to be just a little bit closer together because you think they are closely related.