Many web servers can compress data prior to sending it, using a variety of different compression methods. When a client makes a request of a server, it includes the Accept-Encoding header which informs the server which encoding methods the client supports. WebCopy supports the following compression methods
Disabling compression will mean the Accept-Encoding header will not be sent with requests, informing the web server not to compress content before serving it.
Some servers may not support all available compression options. If you receive 406 (Not Accepted) errors when trying to copy a website, specifying an unsupported encoding could be the cause.