Customs services in angular mean you can create your own services or you can create your own unit of code which can be used in all module or controller. Custom services mean you write your own reusable code.
Services is a unit of code which can be used across the project controllers. Means when we create a service we can use it in any controllers of our project. Services required when we need to use the same code in multiple places. Services can be utilities or business logic unit which means whenever you write any kind of code as part of a service you had to consider that code to be almost independent of that particular controller. It also some business logic code.
Why services –
1-Services provides lots of built-in functionality
2-Best place to implement reusable code
3-Clean and maintainable code