Brake Cable Ends - Barrel and Pear Nipples
Original MTB brake lever designs did not include a rotating barrel inside the lever
to account for the change in angle as you apply the brake, so the wire end needed to be round to accommodate this. 

Road bike brakes have had the interior rotating barrel for a very long time. 

If the wire end does not rotate, the brake wire will be flexed in a "coat hanger" 
way and break in a relatively short time. 

They are two different solutions to the same engineering problem.