Explain why it is vital to produce a process model of the current business, before designing the future business processes?

Business Process Modelling techniques are concerned with 'mapping' and 'workflow' to enable understanding, analysis and positive change. Diagrams - essentially 'flow diagrams' - are a central feature of the methodology.

As Mr. Vinod explained Process Modeling is about analyzing and illustrating business processes.

What I would like to add here is that business model is also about  managing business processes so it maps out the current situation and create baselines for improvements and it designs the future  processes after improvement.

Therefore future processes are based on the current situation, and you cannot start working on future plans or processes before knowing and assessing your current situation in order to improve any "unbalanced" process.

I fully agree with MR Vinod Jetley ..................  thanks.