The real difference in plain words
WhatsApp Business app is free, limited to one device, requires every message to be sent by a human, one conversation at a time; WhatsApp Business API is a paid solution working across devices, team members and entire messaging ecosystems, capable of being automated to deliver responses, broadcasts and orders with no human input needed. Both have their purpose, but they serve different masters - the right choice depends on the current needs of a particular business.
Other differences
Across Users
The business app allows for one device to be authorised as active at a given time (with an option to sync with a second one as a backup), whereas the API allows for unlimited seats for agents to access the same WhatsApp number.
Automation
The business app has no automation beyond pre-set away and greeting messages; the API can utilise bots to communicate with customers in a human-like manner, responding to their individual requests and questions.
Broadcasts
The business app has limited options to broadcast messages to lists of contacts based on WhatsApp’s segmentation rules (but these are limited to only those who have saved your number and can be unreliable), whereas the API has options for large-scale marketing blasts limited only by the budget (as conversations count towards the sender’s rate plan).
Costs
The business app is free to use, but the API requires a paid plan - however, businesses using a WhatsApp Business API solution typically see ROI from the sheer volume of orders that would otherwise be inaccessible to them.
Verification
Only businesses using WhatsApp Business API can be verified by Meta for the green tick display next to their name.
Catalogue and commerce
The business app has no shopping features aside from a static catalog, whereas the API provides full-end commerce solutions, from the chatbot-guided ordering process to the ability to push products directly to the customer and sync orders with external systems.
Integration
The business app has no third-party software integration beyond WhatsApp itself; businesses using the API can connect WhatsApp to their CRM, order management system, payment processors and other software to enable fully automated end-to-end conversations without any manual input required.
When the free solution is sufficient
A sole proprietor with a small shop and little to no customers beyond their local area, with messages averaging fewer than 40 per day, with no expectation of replying to messages in real-time and no need for mass communication features.
When the free solution stops being enough
There are more customers, but their messages go unanswered for days because the sole proprietor has no choice but to respond manually one by one.
The business owner begins to miss out on sales because customers don’t get the answers they need in a timely manner.
It becomes necessary to let customers know about a special holiday promotion, but there is no way to announce this to all of them without calling everyone one by one or risking being marked as spam.
There is a need to integrate the WhatsApp orders and inquiries with external software in a way that requires no manual input beyond receiving the message itself.
The hidden costs of the free app
The cost of the business app is right there for anyone to see - it is zero, period. However, businesses that use the free version of WhatsApp Business bear the opportunity costs of the leads they waste on unanswered messages, the promotions they cannot deliver to all of their customers at once, and the hours spent typing individual order confirmations and replies instead of focusing on running their business.
What the article says about which option is better, applied to real-life scenarios
Which option is right for a given business depends on the number of messages sent per day and the amount of revenue wasted on slow replies or unfulfilled communication needs. It is a matter best decided on a case-by-case basis.