Joining two connected thoughts with a semicolon is exactly the same grammatical form as joining two connected thoughts with a comma and one of the FANBOYS. These two sentence structures can be used interchangeably. So how does a writer know which one to choose for each particular sentence? It all comes down to sentence style and the flow of the writing.
Using a comma and one of the FANBOYS creates a longer-feeling sentence. This style feels connected as you are joining one thought to another with the conjunction, and it creates a smooth flow to the writing.
It’s raining today, so the game will be canceled.
The semicolon, on the other hand, creates a bit of a hiccup in the writing. Use a semicolon to connect two related thoughts when you want that more abrupt feeling to the sentence. The semicolon causes the reader to pause just a bit when moving from one idea to the other related idea, and that pause perhaps puts a bit more emphasis on the fact that these are two distinctly different thoughts, even though they are related.
It’s raining today; the game will be canceled.
So to recap: use a comma and one of the FANBOYS when you want your related thoughts to flow seamlessly from one to the other in a single sentence. Use a semicolon if you prefer to emphasize the separation between the two related thoughts. The comma/FANBOYS creates flow in the sentence. The semicolon will force the reader to pause a bit between the related thoughts.