Why use View Components and not Partial Views? The biggest reason is that when inserting a Partial View into a Razor page, all the ViewData associated with the calling View is automatically associated with the Partial View. This means that a Partial View may behave very differently on one Razor page than on another. With View Components, you control what gets shared to your View Components. 

Partial Views and Child Actions are one the most used features of ASP.NET MVC. Partial Views provides us a way to create a reusable component that can be used in multiple Views. There are Actions which can be marked as Child Actions and these cannot be invoked via URL but inside views or partial views. Child Actions are no more available with ASP.NET Core. View Components are new way to implement this feature in ASP.NET Core.