For the techincally savvy who understand cabling and if IDE drives proper jumpering this is preferable because USB is slower than directly connecting the drive internally.
the rub is that you must power down to connect/disconnect the drive.
a USB enclosure allows you to simply switch the drive on anf off at will.
BTW, on "powered enclosures" saying an externally powered enclosure for a 3.5" desktop HDD is a redundant statement, as there is no other kind..
For Notebook drives? Yes there are both powered and unpowered, but the older style seperately powered kind are RARE nowadays
and used the SLOW USB interfacce.