@Samy Abdul Azure Functions is like regular backends in many ways and their main differentiation is being elastic. Especially the Consumption Plan allows your function app to scale in/out based on the load with you having to pay only for what is used.
This characteristic makes it the perfect fit for use cases which have activity at certain times. So, when nothing is running, you aren't charged for anything (well apart for storage which is very minimal in most cases).
Contrary to what you've mentioned, small projects benefit the most and could almost cost nothing if the number of invocations are less than a million per month.
I believe this architecture doc on serverless functions should be an informative read to understand more on how you can fit functions into your design.