API Testing can be used for three main purposes, viz., validation of an application, maintenance/upgrade of a system, or elimination of errors and defects in software.
The primary benefit of API Testing is that it enables developers and testers alike to gain access to an application, without a User Interface or getting into the internals of the system. It is extremely beneficial when dealing with disparate systems about which specific knowledge is lacking or unavailable.
Secondly, as API Testing occurs before the UI Testing phase, and occurs at the Application Logic Layer, it enables development teams to detect errors fairly early on in the Software Development Life Cycle, thus preempting them from escalating into full-blown bugs in the software. API Tests provide an early evaluation of build strength.
Thirdly, API Testing is much faster and less time- and resource-intensive than Functional UI Testing. Generally the scale of API Testing is 25–30x times faster than the corresponding GUI Testing for the same application.
Fourth, API Testing is language-independent, and exchanges data using XML or JSON, which enables you to use any programming language for conduction of the API Tests.
Last but not the least, by using Automation Tools, API Testing can be speeded up considerably, leading to significant cost savings as well.