Portable AC units are a great way to get cooling on-demand. This is useful in an area of the home or office that doesn't receive adequate cooling (or at least adequate in the opinion of the person occupying the area). It is certainly a less expensive option (at least up front) than moving to a larger air conditioner unit with a higher sq ft capacity, or doing something more extreme like re-working duct lines (though most central air systems should have proper ventilation).
Of course that would only be an option for a home owner. Home renters would not be so fortunate. That of course also applies to those who lease office space, or workers within that space. That's why portable AC units are so convenient.