Deep Heat Lines for spa?

Can someone tell me what the deep heat lines are for in a spa? My plumber is giving me the option. From what I understand it is a 3 way valve that diverts heated water from the heater. But how and why? This is for a gunite spa only with no pool.
THANKS!!!

The only thing I can think of is that water is being diverted to the bottom of the spa instead of the walls of the spa. Hope this helps

Does this help much for an oversized spa? Sounds like it would be minimal?