Where can we find use cases or examples for implementing Lightning Web component
There's a great section called Sample Gallery on Trailhead where you can check different projects and use cases made with best practises in mind, provided by Salesforce.
You can refer to component reference library which has sample codes for all types of components, input types, Grid styles etc. Here is the LWC Component reference. There is tab next to component reference for developer guide.
The Component reference allows you to view the same component code with Aura component by simply click on Switch to Aura Component button. For example below snapshot shows Breadrumb of Aura component. When you click on View as Lightning Web Component it will show you the LWC code. Please note these are not fully developed use cases however you can refer them and learn it.