Azure functions are serverless functions. You can build the Azure Function in various languages like Node.js, C#, F#, Python, PHP, and even Java. Azure Fuctions are event driven which will run based on events or triggers. Also azure functions can be testable without publishing it.
For Example:- I have a Twitter Account and when some tweet me then azure functions will automatically trigger an email to my mail id.

Follow these links for more detail,

https://dzone.com/articles/when-to-use-logic-apps-and-azure-functions

Thanks

if you dont worry about platform or infra.
if you want to avoid cost, because Azure provide build in services these are plug and play no need to develop these things, like azure monitoring, exception handling, monitoring
if u go for microservice based archetecture then Azure is very managed plateform
if u want to automate your work then go to Azure, rather then giving 24*7 support of an application
disaster recovery , replication, cdn etc use As PAAS
Pay As you GO is the plus point.

https://docs.microsoft.com/en-us/dotnet/architecture/serverless/serverless-business-scenarios