why put services in constructor angular code example

Example 1: make service in angular 8

For Creating Service you have type command like below
ng generate service service-name
or
ng g s service-name

And if you want to make service into folder
ng generate service /folder-name/service-name
or
ng g s /folder-name/service-name

Example 2: how to inject service in component angular 6

you can inject service in components constructor:
constructor(public anyService: Service){}

Example 3: angular how to use service in class

import { <service name>Service } from '../<service path>';
you can inject service in components constructor:
constructor(public anyService: Service){}