That's a great question. I have noticed the question of Workflow vs Visual Workflow (Flows) in their recently published book, "Practical Salesforce.com Development Without Code (http://amzn.com/1484200985" target="_blank).". Here's an excerpt that addresses your inquiry:
 
Why Visual Workflow?

While Visual Workflow clearly offers more functionality than workflow rules, it would be shortsighted to always use it in place of workflow rules. Workflow rules allow you to create event-triggered actions that occur immediately or at some specified point in the future. While they may possess some level of complexity, managing and/or enhancing them is often a relatively straightforward task. Although possible, using Visual Workflow instead of workflow rules to update Field B when Field A changes would be overkill. While I feel obliged to provide this warning to not exclude workflow rules, it is true that Visual Workflow does cover a number of scenarios that cannot be satisfied by workflow rules alone. We will look at some of these scenarios in the following sections.

Event-Triggered and User-Triggered Flows

Until recently, a primary factor in the decision between using workflow rules and using Visual Workflow in order to employ an automated solution was whether you wanted an action to be driven by an event (as it would be with workflow rules) or by a user (as happens with flows). In the absence of writing custom code to supplement a configuration-based solution, actions resulting from workflow rules continue to be limited to event-triggered activity. A user doesn’t manually make the decision to “activate” a workflow rule; rather the user’s behavior is subject to the configured business rules and his system actions determine whether or not automation occurs.

Visual workflows can be triggered by either a system event or a user action. Previously, visual workflows sat opposite from workflow rules, requiring the user to initiate the process. However, Salesforce.com has delivered the “trigger-ready flow,” changing the game quite drastically. Along with the traditional types of actions that can be driven by a workflow rule (e.g., Email Alerts, Field Updates, Tasks, and Outbound Messages), the trigger-ready flow provides a configuration-based means to kick off a visual workflow with no human intervention. The main example I’ll use in this chapter is a user-triggered flow, but I would definitely recommend researching flow triggers for workflow if you have the opportunity.

User Input and Decision Points

With workflow rules, once the initial user input kicks off a rule, all remaining actions are automated; no additional user input will be elicited. One powerful feature of Visual Workflow is the ability to capture user input during the workflow process. The input can be tied to a particular field or not. For example, you could use a flow to “fill out” a customer’s Contact record in a logical or strategic sequence that differs from the standard Contact detail page. Alternatively, you could ask questions that would help determine how to handle a particular customer situation. Specifically, these questions could be associated with one or more decision points, another standout feature of Visual Workflow. Using either existing data or data captured during an executed workflow, you can configure different outcomes to occur.

Screen Output and User Interface Impact

With workflow rules, the only notable impact on the user interface has been the automated changes made to the modified record. However, with Visual Workflow, new custom pages showing customized combinations of text and existing data can be displayed to the user through the Salesforce.com user interface. These pages could be used capture user input, as I previously mentioned, or depending on your organization’s needs, they could instead be used to display useful information to the user. For example, a customized set of fields showing key contact, account, and subscription information could be displayed to aid the user in confirming a customer’s identity.

Multiple Steps / Cohesive View of Process

Although it is possible to “chain” more than one rule to trigger an action when using workflow rules, doing so adds significant risk and complexity to your system’s behavior. While some use cases may exist that could benefit from chaining workflow rules, the sensitivity of this approach prevents me from recommending it as a broad approach for a solution. The fact alone that your chained rules have no direct association that is visible within the Setup menu is a potential challenge that warrants thinking twice before utilizing that approach.

Visual Workflow, on the other hand, is ripe for multiple updates or steps within an automated process. The “Visual” aspect of this tool provides a major boost. If you create a sequence of updates within a flow, you can get a visual representation of the process and see what happens along the way. Additionally, the flow is designed for these sequential updates. Workflow rules are very limited in this regard, in that a workflow rule-driven action that modifies the originally updated record can’t trigger a second update to that same record. With Visual Workflow, you can update the same record multiple times without an issue.


For further reference, you can check this too,

https://help.salesforce.com/articleView?id=flow_vs_workflow.htm&type=5

I hope you find the above information is helpful. If it does, please mark as Best Answer to help others too.

Thanks.