The SharePoint 2010 query server accepts the query from an end user from the web front-end. It then reads the index file and servers the result back.
The index file is partitioned. As query servers are added, the partition is broken into multiple parts so that no single query server holds the entire index. The following screenshot is from the Managing Search Topology page:
It illustrates how the index partition and query server are tied to one another. The index partition is sent to the query server by the crawl component. As can be seen from the preceding screenshot, each partition must have an associated query server.
It enhances the performance of the topology. There is no longer one large index file that must be traversed every time a query is sent.
You must have farm-level administrative permissions to the Central Administration site. There must also be another server on which to host the query server.