Because if you don't it will take longer to sell and you will probably end up selling it for a lot less than it's worth.
You'd be amazed at how seemingly trivial things can increase the value of your home if you get them right — and cost you money if you don't. When your prospective buyer sees your house for the first time they must begin to fall in love with it. If there's no emotional attachment there's no sale. Your buyer must like what they see and be able to imagine how the house will look when it's theirs.
If the first thing a buyer sees is an untidy front garden and paint peeling off the front door they may not even bother to view.
Fortunately, staging your home well is not difficult. It just takes a bit of planning and effort. But remember, you've got to clear the house anyway before you move, and chances are you'll more than recoup the time spent preparing by getting a quicker sale.