A x.5 way mates an x-way speaker with an additional low frequency driver that only has a high pass filter. It lets the designer compensate for baffle step loss without giving up 3dB or more in sensitivity.
It is not possible to build a conventional 2-way speaker that covers the full audible spectrum, supports realistic output levels in the bottom octaves, and doesn't suffer from beaming at high frequencies.
A separate sub-woofer counts as a third "way" with the added advantages of simplifying placement (you can place it for best bass response, which will happen in a different location than what provides the best sound at higher frequencies), lowering cost (at least to the manifacturer - you can have a single driver instead of a pair, and power amplifiers cost less than the large inductors/capacitors needed for passive low frequency cross-overs), and increasing output (because it can operate into a smaller acoustic space than a floor standing speaker placed away from room boundaries).
It is not possible to build a conventional 2-way speaker that covers the full audible spectrum, supports realistic output levels in the bottom octaves, and doesn't suffer from beaming at high frequencies.
A separate sub-woofer counts as a third "way" with the added advantages of simplifying placement (you can place it for best bass response, which will happen in a different location than what provides the best sound at higher frequencies), lowering cost (at least to the manifacturer - you can have a single driver instead of a pair, and power amplifiers cost less than the large inductors/capacitors needed for passive low frequency cross-overs), and increasing output (because it can operate into a smaller acoustic space than a floor standing speaker placed away from room boundaries).