I'm building a POS(Point of sale) web application using Django for my client. This is a single instance web app.
I just want to know, should I build APIs to separate Backend and Frontend?
Whare are the benefits to build the APIs?
First and most importantly, each design decision you take for your application should be for a reason. There is no good or bad approaches, however, the approach you decide on should be based on your use-case.
I don't see any problem for having a monolithic Django application to service a single/multiple POS with limited number of users. Tens of thousands of Django applications out-there not using APIs and performing well.
That being said, if you are developing a POS solution and planning to offer it to public as SAAS (software as a service), then you might need to consider using APIs, have plans for scaling/support/patching/monitoring/upgrade/ CI and CD activities. On the other hand, if might be planning to have a Mobile application in the future; That might be another good reason to have APIs.