Another good reason to use a combiner is to preserve impedance matching.  Running two amplifiers in direct parallel connection will not maximize the available power.

 

It can also reduce the risk of damaging both amplifiers if (or more accurately in the real world, "when") one loses drive, or its power supply quits, or other failure occurs.  In commercial practice, we would never dream of strapping two amplifiers together without a combiner that provides some isolation between them... or at least employing a circulator in line with each output, which is not very practical at LF.

 

John