Why would you consider a 44mm head tube?
The most obvious reason is if you are using a tapered fork because, well, you have to. However, there are benefits other than compatibility. For starters, the larger steerer helps combat the additional torque that a disc brake creates thus, improving braking performance. In addition, adding a larger diameter steerer tube and head tube to a bike will add considerable stiffness to the front, it will feel very responsive. Racers, larger riders, and people interested in the stiffest components will love the benefits. Not to mention 44mm headtubes will work with 1 1/8”, 1 1/8” to 1 1/4”, and 1 1/8” to 1 1/2” steerer tubes when paired with the appropriate headset. Another thing worth mentioning that I think of as a builder; the adjoining tubes (downtube and toptube) can now be larger because generally the tube that gets mitered is smaller. Funky things can be done as a work-around but like I said, "funky things"
Why wouldn't you want a 44mm head tube?
Not everyone wants or needs the stiffest frame possible, and some might find the over-sized head tube yields a ride that is too jarring. Smaller riders, and riders who spend day after day on the bike might prefer a more forgiving 1 1/8” head tube.
What headset should I order for my Machine with a 44mm head tube?
Here is a list of popular headsets that work with the 44mm head tube, listed by fork type: