Nearly all web acceleration features of load balancers can be done on your web server, but they do give you a single, hardware-enabled place to do it.  Browser supported compression such as gzip is a nice feature because there’s no additional hit to your web servers. You also get many other options based on content  type and sometimes even speed of remote client.  Caching control transparently enhances remote browser handling of images while freeing staff from maintaining cache headers for all images.  Finally, even TCP flow enhancements are more easily handled (sometimes with more options) on an appliance than tuning the OS of your web servers.