The gal in the photo is working on corn pollen. If you watch a bee closely, you will see that she sticks to one kind of pollen on each trip out of the hive. Thus, this bee working on corn won't stray into the zucchini blossoms or sunflowers.
Also, it is unlikely you would be stung by a bee if you are just watching her working in the garden. Bees tend to sting when they feel their hive is under attack. It's wasps that sting wantonly. If you don't know the difference, be sure to do some online research or get an illustrated book out of the library.
Bees certainly help make the plants grow. They are busy all day long, asking nothing except that you be careful with pesticides, which can poison their honey supply that they feed on.
My favorite book on bees so far is Plan Bee, by Susan Brackney.