APi's are pieces of code that allow other programs to interact with yours. The internet makes full use of this technology by allowing one site to access information from another so that the user has a simple yet meaningful experience.
Take for example a real estate company wanting to have an interactive map of their property. Rather than build out an entire solution themselves, they simply use some simple APIs to connect to a provider like Google Maps and are able to rapidly deploy it, thus saving time and money. Another example is how travel agents are able to arrange your travel plans with airlines, hotels, car rental agencies, etc. from a single application.
API's add value to your website by allowing other websites and applications to easily interact with you without having to build complex solutions for each client. API's are cross platform friendly so you only need to make 1 API that can be understood by different platforms. Android users, Apple users, and Windows users all have the same experience because they all use the same API to connect and access your information.
Connect to cloud based applications
Build a custom solution that isn't already on the market
Access old data or systems
Unify connectivity between applications
Expand your services into new markets
Make your application mobile
Grow sales with your partners
Develop new business solutions