If you want an easy comfortable ride outdoors because you have muscle spasm issues, pressure sore problems or just because you prefer less vibration it can help (a little) to run with a lower tyre pressure that the manufacturers "recommended" pressures. Its your choice! This is usually chosen to be about 30 to 35 PSI depending on who manufactured your power wheelchair. (Or wheelchair)

But they don't even know your weight!  That makes a huge difference all on its own. Adults go from very small and light (say 100 pounds or less) to well over 300lb.  

So even if there was a "correct" pressure to use, it still wouldn't be correct for most people!  If you double the weight a tyre has to carry, then you have to double the pressure to maintain the same tyre shape and contact patch area.  This is one of the many problems of foam filled tyres. No adjustability for user weight. Take a look at the door pillar on your car. More load? More passengers? Requires more pressure. Its the same in your powerchair.

And they don't know your condition. Some people cannot cope with simple vibration or the kind of shocks that hard "correctly" inflated tyres give on our uneven streets for a large bunch of medical reasons. "Suspension" as fitted to current Powerchairs just doesn't cut it. It can "help|" with the big bumps but does nothing to help with smaller high frequency bumps/vibrations such as you experience on cobbles or similar sized bumps.

It can help slightly on bigger jolts but we travel much too slowly for it to ever be very effective. Our wheels are too small! So for these people effected (and myself included) running with lower tyre pressures helps. 

Related Note: I actually removed the (useless) suspension and fitted special fat really low pressure tyres instead to my own powerchairs eventually!   The real problem is that powerchair manufacturers fit tyres with too small sidewalls for comfort. But I digress. (Here if you are interested)

So what other effects does running with lower*** tyre pressures have?  (***lower than advised)

a) causes slightly faster tyre wear generally but over a wider area of the tyre rather than just the middle.

b) causes more grip on snow or slippery conditions (but not hugely so, as the tyres are too small)

c) increases rolling resistance and so shortens range slightly while increasing the average battery discharge level. That means your batteries will die a little earlier. But not by much.