A "Workflow" in Open Leads is basically a macro, like you would use in a spreadsheet. It is a single process that apply's several actions to a single lead.
For example, a Workflow can be configured to assign a user automatically via a round robin rotation, and send a "new lead notice" email to the assigned user; assign a "Type" to the lead; send an "Autoresponder" email to the lead; and schedule a followup task to the lead, for the assigned user - all at once.
Having a workflow to handle these individual, separate steps saves a ton of time, and can be used as an automator for incoming leads generated via a weform, applied manually to a lead, or triggered automatically when a task is completed.