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Many of these backend systems will be Off-the-shelf products, e.g., SAP, Salesforce, Marketto, and lots of other systems people use. Many of these systems don't have APIs that work well for interactive frontends. And you have many different frontends, for your sales people, your project engineers, your marketing department, your different suppliers and partners. These different apps have different data & api needs, and will need to aggregate data from different systems. Systems that cannot be modified to execute the most optimal SQL query for your frontend. And it's too much business logic to put in an API gateway. And all these systems have their own lifecycles, so different project timing, different releases, etc. A BFF makes a lot of sense to make all of this manageable.