Using the ledger's API allows (you/others) to (make/use) connected services which utilize the ledger. Below is a visual represenation of connected services:
Ideally as the ecosystem develops, connected services become inner-connected:
The aim of any of these services is to provide functionality which relies on the ground truth of ownership of currency. This information is centrally secured in CCash.
To browse the currently available connected services, check out existing_services.md.
To make a connected service yourself, you can do so by using one of the langauge specific APIs. What they do is provide a simplified in-langauge way to interface with the CCash instance.
If an API does not exist for your language, it is simple to make one. I encourage you to add it to this list if you do decide to.
While developing make sure to reference the list of endpoints.