Well the only way I can really explain is with a picture
these are the internal cables that connect to your motherboard. there would be no point connecting on the front of the unit via esata, only to have it connected at the back via usb2.
generally, esata is better for hdd/ssd as there is no need to convert to usb, then back to sata. the whole stream remains in sata. (geeks seem to think) this is better.
Sata has two speeds. 3Mbps and 6Mbps (note these are transfer bits not storage bytes). You will need to have 6mbps sata ports on your motherboard to be able to use these with the device i showed earlier, and most likely the little reader will only be 3mbps, unless you manage to find a 6mbps one. The other alternative is to get a pci express 6mbps esata controller card, but then you'll have to plug in at the back of your machine instead of the front (and if you only have one pci express slot on the motherboard it is probably taken up by the video card).