The first and most important purpose is to create a barrier between your electrolyzer or reservoir and a possible flashback. The general concept is simple. The HHO is introduced into a fluid column through a barb at the bottom.
The fluid column usually consists of water. This causes the HHO to break up into bubbles and rise through the fluid column to the top where it is collected. This mechanism creates a water barrier between each HHO bubble that provides a heightened level of safety.
Though this device will stop most flashbacks, one must be careful with the size of the fluid column and how much HHO is introduced into the liquid. If the HHO is introduced too fast into a fluid column that is too small, large bubbles will form, reducing the water barrier between each.
This scenario could possibly allow an ignition to cause each bubble to ignite the one next to it and travel past the fluid column and into your reservoir or Wet Cell container. This is called a Daisy Chain ignition.
Proper care should be taken to break up the bubbles as much as possible. One solution consists of a sintered (porous) metal element that acts as an air stone, breaking up the HHO into tiny bubbles. Since it is made of bronze, it resists corrosion and can't break apart like other designs.
Scrubbing
The second purpose for a bubbler is to provide a scrubbing action. This is also called sparging. By introducing HHO into a fluid column in smaller bubbles, we provide more sufrace area for the water to "scrub" the HHO. This means the water helps absorb any electrolyte condensate or other chemicals that might make it past the reservor and into your device. This occurs because the lighter HHO gas is pushing its way to the surface. As the bubbles rise, electrolyte condensate (which is heavier) will fall to the edges of the bubble, and be absorbed into the fluid. This mechanism helps provide you with clean HHO gas.