I'm not fond of regimented vegetable gardens so adding flowers feels natural. Sweet alyssum spilling over the edge of a bed softens the line, Larkspur among the cabbages gives some height. They all shade the soil.
And, of course, annuals are naturals in pots and any bed that's a little sparse. The best thing about them? They grow quickly and they (usually) flower for months. Perennials have years to build up their strength, their root systems and their bulk. Annuals have this season and that's it. So they get going fast and put out lots of flowers to ensure a good crop of seed. Result - a colorful garden fairly quickly.
Some annuals are short lived, especially the hardy ones that get going early in spring. To keep them around you'll have to seed them several times. And some will stop blooming as soon as any flowers set seed, so you'll have to cut off the spent flowers to trick the plant into putting out more.
I'm not fond of annuals as the sole inhabitants of a garden but they sure make good companions!