Each time a change in the system's data occurs, the SQL Server log this change in its transaction log, regardless whether it is a SQL Server on your network or a SQL Server Express edition.
The transaction log is essentially a security feature that makes it possible to roll back and undo changes to the SQL Server database. By default, the SQL Server stores its transaction log indefinitely, and over time the transaction log build up more and more entries. The SQL Server's transaction log is by default located on the system drive, and if the transaction log keeps growing, it may in the end prevent Windows from running properly.
To avoid such a scenario, flushing the SQL Serve's transaction log from time to time is a good idea. However, flushing it does not in itself make the transaction log file smaller, but it prevents it from growing out of control. Your system does not, however, automatically flush the SQL Server's transaction log at specific intervals. You can also do several things on the SQL Server itself to keep the size of the transaction log down.
For more information on this topic, go to the Microsoft support page and search for SQL Server transaction log.
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