Using annuals allows you to choose something new and different each year--whether for your garden space or your flower bed. Unlike with perennials, there are no roots left in the soil to disturb; the soil may be tilled and amended without worry about damaging a dormant plant. Thinning allows you to select the best of the batch of seedlings and avoid many common gardening hassles.
Benefits for Plants
Thinning creates space between seedlings, allowing air flow. Good air circulation allows excess moisture to dry and reduces the potential for disease. It reduces competition between plants, leaving more nutrients, sun and water available for the plants left behind. Keeping a smaller number of plants per row reduces stress on the remaining seedlings; pressured seedlings may develop leggy features or fail to thrive. Transplant seedlings if you have extra room or cannot bear to kill a thriving plant.
Benefits for You
A willingness to thin allows you to broadcast seed and save a bit of time in the early stages of planting. Gardeners do not have to worry as much about achieving perfect spacing when planting rows. Instead of fussing with very tiny seeds (notoriously difficult to space evenly or separate properly), disperse them generously over the soil and thin after germination. Eat the discards; if you grow vegetables, the University of Nebraska-Lincoln Extension recommends waiting until the thinned plants are big enough to eat. You can add them to a salad and none of your hard work will go to waste.
Benefits for the Garden
Thinning allows you to plant more densely, initially. Instead of having holes in your garden space where seeds failed to germinate, you will have an abundance and can be assured that enough plants will survive to cover the bed.
Thin plants in place to tailor the shape of your flower bed. For a curved edge, simply remove all of the plants which fall outside of the boundary you want to define and then select the best plants within the ideal spacing distance (between plants). Choose the healthiest specimens from your sprouted seeds; larger, better developed seedlings will most likely grow to be the strongest plants.
If you are close to the last frost date or experience harsh weather conditions, planting more seed than you need (for thinning at a later date) gives you better odds that some plants will make it through. Thin any damaged plants and allow the tough survivors to grow into the gaps. Thin very closely spaced plants (without harming the roots of the plant you intend to keep) by using a pair of scissors. Clip off the plant you want to thin at ground level.