When all the data has been fetched from a SELECT statement, the driver should automatically call finish for you. So you should not normally need to call it explicitly except when you know that you've not fetched all the data from a statement handle. The most common example is when you only want to fetch one row, but in that case the selectrow_* methods may be better anyway. Adding calls to finish after each fetch loop is a common mistake, don't do it, it can mask genuine problems like uncaught fetch errors.

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Calling finish resets the /Active attribute for the statement. It may also make some statement handle attributes (such as NAME and TYPE) unavailable if they have not already been accessed (and thus cached). 

The finish method does not affect the transaction status of the database connection. It has nothing to do with transactions. It's mostly an internal "housekeeping" method that is rarely needed. See also /disconnect and the /Active attribute.

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If execute() is called on a statement handle that's still active ($sth->{Active} is true) then it should effectively call finish() to tidy up the previous execution results before starting this new execution.