External Hardware Summing?

As Bristol sez above.
 
Summers became a thing since many pro mixers received songs with many tracks or stems and had nice hardware at home.  But not a big Neve or SSL but they still wanted to use their neve EQ or LA2A etc. So to integrate the hardware they got simple line mixers to run the tracks/stems through and used the hardware they were used to using.  They didn't need a $100,000 mixer with every feature on every channel, but they don't want any cheap components in line either.  So clean, simple Summing mixers came into being, no compromise line mixer with maybe some monitoring features.
 
Another thing was Protools lower mixing bit rate.  Early PT hardware was limited to 24 bit internal mixing, I believe, when SONAR used 32 bit (internal) mixing.  My TC unit claimed 48 bits.  And when PT moved to 32 bits, SONAR was using 64 bit processing.  (my timeline may be off but the thrust is correct).  Some people found that mixing out into a board from PT got them a less strained sound than using the low mixing bit rates to mix in PT internally.  Since so many people use(d) PT it made economic sense to come up w/ external Summing units, even if you didn't have any fancy pants external hardware to use with it.  In that belief system, the more summing tracks the merrier (and less summing in PT), where 16 tracks and more were required.
 
What you are talking about is "mixing" a stereo out into external, analog hardware.  That would change the sound of your stereo track, so you would need to send it back into SONAR to record the new tone stamped on your track.  I do exactly that when mastering (note: I don't mix using the hardware).  But many people do mix through hardware (or software) on the two-buss - just be sure to capture the post fx sound.