Why is Cache-Control attribute sent in request header (client to server)?

A client can send a Cache-Control header in a request in order to request specific caching behavior, such as revalidation, from the origin server and any intermediate proxy servers along the request path.

In addition to the above answer,
There might be a setup where cache chaining is implemented. In that case if the request comes to first cache where it is not satisfied, it might go to further chained cache.

Thus in order to get the response always from the server we include cache-control in request headers. This will insure that response is always from the server.

Cache-Control: no-cache is generally used in a request header (sent from web browser to server) to force validation of the resource in the intermediate proxies. If the client doesn't send this request to the server, intermediate proxies will return a copy of the content if it is fresh (has not expired according to Expire or max-age fields). Cache-Control directs these proxies to revalidate the copy even if it is fresh.