My mother always told me to have at least two pairs of shoes to wear alternately. Why is that?

Shoes, particularly leather shoes, regain their shapes when allowed to ‘rest’ for 24 hours.  By allowing the shoes to naturally re-shape themselves, they will last longer.  Also, any time you have a removable insert inside of a shoe, when you take the shoes off, take the inserts out and let them air dry separately.  Perspiration breaks down the interior of shoes, so allowing air to circulate, you can increase the lifespan of them.