I have used only Spring framework and deployed as spring boot application It just opens and runs as a java application , Why do a companies with only spring framework runs on app server they can run on JAVA application as usual ? why do they need App server? What all can an app server does ?
Application servers are usually designed to host multiple applications, and manage a set of services that are used by all these applications. These services might include transaction management, timers and task management, HTTP request routing, a message broker (for inter-process communication, among other things), user management, etc. There's usually a graphical or command-line management console, or both.
The Spring framework is usually used to build a single, mostly-self-contained application. Spring does provide common services like transaction management, although they typically require a deal more developer understanding than is the case with, say, a JEE appserver.
There are all sorts of application containers and frameworks, offering different kinds of services in different ways. Often there is no killer reason to pick one over the other, and they are to some extent interchangeable. Spring Boot seems to be rising in popularity right now, because (perhaps) of its better fit to the microservices-type development models that are currently popular.