In the ported box, the port itself produces some of the sound. This
required less actual work on the part of the sub. In a sealed box, the
sub itself is responsible for more sound, so to get the same level of
volume, more power is required. The advantage of a sealed box is usually
a flatter response curve and higher power handling. Of course, a well
built ported box can give you a reasonably flat response curve without
requiring lots of power, but then they tend to become "unloaded" at low
frequencies.

Thats the sub's low end extending down into lower frequencies.  Many ported
designs produce little output below the port tuning frequency.  I usually
prefer sealed enclosures for this reason.

Garrett