output buffering / output compression

I have recently set up a machine with PHP with both buffering and compression turned on. Some of the pages being served include up to 3MB of HTML. Can someone explain the benefit(s) of setting these ini directives to values (ie. larger than 4kb) rather than "On" ? I've played around with it as I thought it might speed things up by using a bigger buffer etc. but the only difference I noticed is a big jump in memory usage. Thanks!