Purpose is the reason you write. Specifically within engineering, purpose is the goal you want to accomplish with your work. It can include you wanting to explain something to your audience, or asking for funding for your work. Identifying purpose often leads to determining the other rhetorical elements.

In regards to the engineering proposal, our purpose was rather simple. Since heating and cooling systems within cities are a huge contributor to climate change, we decided to tackle that. Because we are either mechanical engineers or computer scientists, we picked a thermostat as it is a device that uses both of our talents. The purpose of an engineering proposal is to be an offer to carry out research or provide a product/service. The purpose behind our writing it was to present an issue we feel strongly about and present it, as well as a solution we felt would fix it in the most efficient way possible. There is a purpose behind the work we create, and then there is a purpose for our doing it in the first place. This is different from exigence, as there is a difference between a reason to do something and the demand we felt that compelled us to do something.