Many administrators always use -z in conjunction with -a to compress the data before transmission:
rsync -az dir host:destination_dir
Compression can improve performance in certain situations. For example, if you are uploading a large amount of data across a slow connection (such as the slow upstream link on many DSL connections), or if the latency between the two hosts is high, it helps. However, across a fast 100 Mb/s local area network, the two endpoints can spend more time compressing and decompressing data than the network takes transmitting the uncompressed files.