As a seller, you have spent time in the home and are familiar with it. You may even be sentimental about it. If you think you're not, watching buyers walk through your home may change your mind. Watching people tour your home is tough on any seller, and inhibiting to most buyers.
Agents strongly encourage sellers to leave their house during showings to allow the buyers breathing room to take a good look at the house. As a seller, you have some protection against theft and damage when an agent takes his or her buyers through your home.
If you're selling by owner you can manage showings in one of three ways:
The more you restrict showings, the more buyers you're going to lose. Many buyers have busy schedules and limited time to see properties. It is sometimes very inconvenient to keep your house clean and welcome people in even on short notice, but if you want to sell your house you have to let buyers see it.