Essentially, Workflows are the main container of the rules governing any automation. They actually replace the part where you had to constantly monitor any reports, or dashboards to know if it is time to make a certain action. They automate the entire process with their rules/instructions that can be summed up in an if/then statement.
In simpler terms, Workflow in Salesforce speeds up the manual processes and saves your time. They accelerate your business processes by eliminating the manual time lag consumed to review the information to decide if any action is required.