This is a great question, there are many ways we can use diagrams and maps to make information easier to process visually.
A flowchart is a diagram depicting a process, a system or a computer algorithm. It is a diagrammatic representation of the solution to a given problem but, more importantly, it provides a breakdown of the essential steps to solving the problem.
When designing and planning a process, flowcharts can help you identify its essential steps and simultaneously offer the bigger picture of the process. It organises the tasks in chronological order and identify them by type, e.g. process, decision, data, etc.
Each step is independent of implementation as the flowchart only describes what should happen at that step, what input is needed and what the output of the step is but it says nothing about how to implement the step.
Once a flowchart is in place, it can help find less obvious features of the process that can then be refined to improve its efficiency, i.e. bottlenecks, flaws, unnecessary steps. It should be seen as an evolving diagram.
Flowcharts are limited in what they can represent and are usually employed alongside other types of diagrams. They do have historical importance. They were first introduced in 1921 as flow process chart to members of the American Society of Mechanical Engineers and were popular in early computing in the 50s. Visual Programming Languages used in computing are sophisticated flowcharts.
There is a brilliant commentary on flowcharts by xkcd:
“What is the most important part of a flow chart?"
Readability is probably the most important aspect of a flowchart. As the diagrammatic representation of a process, the flowchart aims to offer a visual description of a process to help us understand what is going on. However flowcharts can get a bit out of control and you can end up with something so complicated that it defeats the purpose of having one.