Workflows is a event driven platform, So if you have certain scenarios like once you fetch records from calender you want to format it in a particular way before indexing you can do that.
Second let say based on your query you wanna perform some action like cancel calender booking or create a new booking. You can do that in a event.
You can find lot of examples here for workflow implementation:
https://docs.llamaindex.ai/en/stable/module_guides/workflow/#examples