When you are working in a Cisco Lab environment you cant keep on changing the console cable from one router to another for configuration for this purpose an access server is used. Access server has several config cables coming out from a single port. These cables can be connected to different routers in the network. Now while only logging into a single router we can configure all the other routers that are connected using the special cable.
Now this is the case with real routers, when you are using GNS3 as simulator then it also becomes a hassle to manage the console of each router. Tools like puttycm and secureCRT can be used but still these have multiple consoles.
There is one cool way to use a router as an ACCESS server in GNS3