In recent years it feels like combos have exploded in popularity. As the vast majority of venues both large and small now have a PA and microphones for amps, the days of needing two 4x12 cabs seem to be long gone, with players preferring the portability of a smaller combo instead.
For certain styles of music like sludge and doom where volume is a requirement, however, the trusty head and cab combo is a necessity. You might also already have a great cab and want a different head to change things up or mix and match cabs.
There’s also a certain three-dimensionality that comes from using a larger cab, as the sound is reaching you on many different planes thanks to the multiple-speaker layout.