This is a question I ask clients when I am designing a garden. The answers are surprisingly varied.
To relax in, to entertain in, to be in touch with nature, for the children to have somewhere safe to play, to grow our own food in, to let the dog out in, to grow flowers for cutting, to add to the value of the house, to create something beautiful, to dry the washing in, to lie in the sun, to keep up with our neighbours, to feed the birds and encourage wildlife, to grow prizewinning leeks/chrysanths/sweet peas/carrots, to park the car in, to tinker with motorbikes in, for parties/barbecues.
Sometimes all of these.
No, not all! But most people want to use their garden for several purposes - not always compatible.
If you have a garden have you thought about what it is you want it for?
And does it serve your purpose?