Bees (and other pollinating insects) are one of the major reasons why you have plants in your garden. :) They help pollinate plants and trees such as fruit-bearing trees. Without the pollination of the flowers on, let's say, an apple tree, you will have pretty blossoms in the Spring, but no fruit in the Fall. This is the same for all fruit-bearing trees. And with no fruit, there are no seeds, and with no seeds, no next generation of trees. Pollination is vital for practically every plant life form on the planet. Bees play a major part in this task in that they transfer pollen from one plant's flower to another. A good amount of our crops are pollinated by bees and many farmers lease bees to carry out the pollination of their fields and orchards. Without commercially leased bees, the crop yields would be too low for the demand of food that we have for our country.

So in a nutshell, bees are very important to any garden, especially the small gardens where bees are most likely doing a majority of the work. :)

Here is a link on pollination and bees -
http://en.wikipedia.org/wiki/Pollination

Cheers.