Among others, they deliver higher efficiency and a lower frequency response in a smaller cabinet. As with sealed Isobaric, this design helps to reduce the total cabinet size of a design. Band Pass Band pass designs are various. From a fourth order, fifth order, sixth order, these are the most complex to design and the most critical to assemble. The driver itself is not visible.
Sound transmission is done through ports a bit like vented enclosures.
Here's an example of a sixth order bandpass. Having the rear chamber sealed would make it a fourth order. Adding an inductor would make it either a fifth or a seveth order depending on wheter or not there is a port. This type of cabinet is like having a vented enclosure in front of the driver, which inceases the complexity of such designs. A transmission line is basically a large stuffed vent with the driver at one end.