Four great reasons to give when someone says why do you need a new pair of shoes.
Style; it changes constantly we have to keep up.
Comfort; just try playing basket ball in your 3 inch heels! You will need sport shoes, running shoes, dress shoes, flats, sandals and slippers.
Health; it is bad, bad, bad to wear the same style of shoe all the time. You need to flex and relax that Achilles tendon. Women who wear only heels all day can have great pain when they try to wear flats. That poor tendon doesn't know how to work correctly when lengthened. That is one of the reasons why that famous shoe that starts with a B... is so good for you. 
The first few days of wearing them are heck but they conform to your feet and give that tendon the stretching action it needs. (It can make for a shapelier leg too.)
Lets talk sweat; do you wear the same pair of sport shoes day after day for hours at a time? The same goes with business shoes, flats or heels. Ladies we have about 250,000 sweat glands in each of our feet. You need to change up your shoes several times a day just to let the poor things breathe a little. 
The next time anyone says you don't need a new pair of shoes, use one of the above reasons (I especially like reactions to the sweat one,) then watch them help you pick out the new ones.