Wikipedia would tell you. HTTP proxies can give a performance boost compared to SOCKS and other lower-layer protocols. HTTP proxies understand the HTTP protocol, so they can use caching technologies. For example they can pre-download the data you get (a technique used at seo-proxies.com) so the data is downloaded with 100mbps+, this can increase your performance. They are not always secure, actually they are usually less secure. (depends on their configuration)
