Could you give some examples?
Sure. In fact, Zapier have a good example right on their home page, showing a connection between someone submitting a form to Wufoo and adding their email address to a MailChimp mailing list.
For automation purposes, Wufoo can trigger a webhook when a form is submitted and MailChimp have a REST API that can be used to manage mailing lists. So, if I wanted to automate that sort of task, I’d probably just set up an endpoint on a server to receive those webhooks and then write a few lines of Python to extract the required form fields and send them off to MailChimp, instead of introducing the additional dependency and potentially cost of an intermediary service like Zapier.