Under atmospheric pressure, that is the pressure that you are sitting in right now, oxygen transport is limited by the oxygen binging capacity of the haemoglobin in red blood cells and very little oxygen is transported by blood plasma.

Because the haemoglobin of the red blood cells is almost saturated with oxygen under normal atmospheric pressure, this route of transport cannot be exploited any further.

Fortunately, oxygen transport by plasma is significantly increased when using HBOT.  This gives the potential for much higher levels of oxygen to reach damaged or under-functioning tissues. 

With more oxygen being present for sufficient periods of time these tissues are given a greater opportunity for repair and healing.

Put simply, you sit, you relax, you breathe!