Let's say you are developing a software or might be creating a website for your customer. In such type of business there are many cases in which your software might not work like the customer required or may be your website will not give you, your required output.
That is because majority of the software are not made through Software Engineering guidelines. Knowing how the system works is the most important part in software development. For all these purposes Unified Modeling Language (UML) diagram plays much important role. These diagrams tells you how your system will work and helps the programmer to create a system in that manner.
Data flow diagram describes how the system functions will flow in an order. You will give me any scenario and i will create its data flow diagram.