Speaking of the first case, we develop a website with this technology in order to reduce the load on the server and to scale the project more easily. It's not a secret for anyone that the profitability of a business depends not only on sales growth. Customers can buy your product, subscribe to it, but if your costs on server content, development are equal to your sales, or even higher, this could hardly be a success. In order to avoid this, we use custom api development. The huge advantage of this approach is that it greatly saves the budget for the making and support of your mobile application on iOS or Android since we use the common aproach.
By the way, it is important to mention for deliver of websites we use the most popular and top-end technologies: Laravel and React JS. For example, in recent versions of Laravel, custom api development is much simpler, which directly reduces your budget.