Authentication Filter and Authentication as a service

It may be irrelevant question, but I want to know about it, please help me to understand the concept. In my project I have seen that Authentication Filter and Authorization Filter have been used and all the filters have been configured in web.xml file. But I also see that there are micro-services like Authentication as service and Authorization as services have been used. Whenever we make REST call, it calls both Authentication, Authorization Filters, also it calls Authentication and Authorization service. In my current application there are many micro services, application has been built on micro service using Spring, Jersey, Spring Security and others. Help me to clarify my questions.

1. Since we are using Authentication and Authorization Filters, what is the use of Authentication as service and Authorization as service ?

2. By making Authentication and Authorization as independent service, what is the big picture are we trying to achieve ?

3. What is the use of Authentication service and Authorization service ?

The whole point is to know about those Authentication and Authorization as services apart from Spring security Filters.