Tires - handling is a story of it's own here in North America - special in Canada.
Really nobody looks at them like they should. Car dealers? Tire dealers/shops?? Car owners?? I am originally from Germany. Every 2 years you need to get your car through an Government Inspection. Tires older than 5 years? GET NEW ONES! Your tires cracking on the sidewall and police is seeing that... ticket for each tire.
I bought in 02/2012 a 2005 Ford Mustang convertible... 30,000 km on the tires. I ask the dealer to change them because there were 7 years old and the sidewalls are cracked... "NO! They are still goood for 100,000 km!..." Drove my Mustang to the tire shop... "What the heck... they have only 30,000 KM on them.. You really don't need new tires..." Yes I do! Got new tires by myself the same day by another dealer.....
Tire pressure should "always!" monitored because:
Disadvantages of Underinflation
.... An underinflated tire can't maintain its shape and becomes flatter than intended while in contact with the road. If a vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would experience a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.
Disadvantages of Overinflation
An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced. If a vehicle's tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well, causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races run higher than normal inflation pressures. The pressure must be checked with a quality air gauge as the inflation pressure cannot be accurately estimated through visual inspection.....
You can read about it all over the internet if you like.
I have in my garage at home an air pressure tank and a tire gauge and check it myself!
....Advantages of Correct Tire Inflation
Maintaining correct tire inflation pressure helps optimize tire performance and fuel economy. Correct tire inflation pressure allows drivers to experience tire comfort, durability and performance designed to match the needs of their vehicles. Tire deflection (the tread and sidewall flexing where the tread comes into contact with the road) will remain as originally designed and excessive sidewall flexing and tread squirm will be avoided. Heat buildup will be managed and rolling resistance will be appropriate. Proper tire inflation pressure also stabilizes the tire's structure, blending the tire's responsiveness, traction and handling......
Check it out, please...
And the question was..."Why do....."
You answered your question already. -> "....In addition, leave the tire at the maximum sidewall pressure during storage. Move the vehicle periodically to avoid “flat-spotting” the tires."
(I got a Japanese Lancer Evolution 4 in December 2013 for my son. Got shipped from Japan and nobody checked the tire pressure at all. Truck driver picked it up in Vancouver/BC to bring it to Edmonton/AB. Got a call from the storage/shipping manager. "..Your front tires don't hold the air anymore... do not know why... tires are only 1 year old....." The truck driver gave a shxx about the tire pressure, drove around with underinflated tires, bearings in the sidewalls broke and Junior needs on his EVOLUTION 4 four new tires! Shipping company does not cover "stupid behavior" of there drivers. So $1,000 on us to get this 4 tires replaced. This time the tire shop figured out right away that the car was driven with to low air pressure. "You can see the bearings on the sidewalls breaking and destroyed. What morons drove that car...?")