Difference between a flared and not flared rectangular waveguide antenna
Short Answer (tons of books, web stuff):

Yes it provides a better match to free space TEM mode.  It allows you to adjust the gain/directivity of the radiated field as well.  

There are other variants on the wall geometry (exponential vs straight) flare that give improvements as well, but are generally more expensive to manufacture.

The primary reason horn antennas are not used below ~1GHz are primarily because of cost/size constraints.  There are waveguides down to about 320MHz that are about 2' x 1', so a waveguide can become rather large at that point, a horn antenna even more so.  

Since the mechanical dimensions and deformation resistance are critical, manufacturing costs for these sizes are high and applications are mostly reserved for high powered UHF RF transmitter outputs.