Writing MySQL by hand is easy to get slightly wrong — a missing closing parenthesis, a trailing comma before FROM, an unclosed quote inside a string literal. These mistakes usually don't show up until you run the query against a real server, and by then you've lost the context of what you were editing. MySQL Syntax Check catches this class of error immediately, in your browser, by scanning the query text for structural problems before you ever open a database client.
The checker looks at the shape of the statement rather than trying to fully parse MySQL's grammar: it verifies that single quotes, double quotes, and parentheses are balanced and correctly nested, flags trailing commas left behind after reordering columns or conditions, and confirms the statement has a recognizable shape (SELECT, INSERT, UPDATE, DELETE, and similar). Because it's a syntax sanity check rather than a query executor, it never opens a connection to any database — there's nothing to configure, no credentials to enter, and no risk of accidentally running a query against production data.
This makes it a fast first pass before pasting a query into a migration script, a stored procedure, or a code review: run it on a rough draft to catch copy-paste mistakes — like a comma left after removing the last column in a SELECT list — before the query reaches an environment where a typo means a failed deploy or a confusing database error message.