Webhooks are user-defined callbacks over HTTP. They are intended to allow web applications to become more extensible, customizable and ultimately more useful. Many web applications only have a request-based output mechanism: web APIs. They lack an event-based output mechanism, and this is the role of webhooks.
Using webhook can greatly reduce the amount of API calls made. Odoo Webhook is a useful tool for apps that want to execute code after a specific event happens on an Odoo database. You can, for example, set up a webhook that will call a URL on your side of the application when a new product has been created or a invoice has been paid on Odoo. This uses a lot less API requests overall, allowing you to build more robust apps, and update your app instantly after a webhook is received.