In general, the idea behind multiple network-servers and a single application server is to manage multiple regions within a single web-interface and also have the network-server close to the gateways (to limit the roundtrip time between the ns and gateways).
How this is intended to use is that every network-server instance has its own pool of gateways. Each pool of gateways would then have its own MQTT broker, or would have their own prefix.
E.g. for US915 it would be us915/gateway/...., EU868 eu868/gateway/.... and so on.
how does Application server differentiates between Networking servers?
With the above setup, received data is automatically routed to the correct NS responsible for handing the gateway data.
what is the reasoning for binding service/device profiles to specific Networking server and what was the initial aim of such binding?
Since this information is stored in the NS database, the AS database only contains a kind of “pointer record”.