At the end of each day they will consume the center portion of the web, leaving the radial line that the "orb" is attached to. It may be that she consumes the web for the extra protein available from the tiny insects caught in the webbing. Or perhaps its just because webbing is a pricey commodity to create and conserving it only makes sense.
She will begin laying eggs on a thin sheet of silk, then cover them with another layer of silk. Then she wraps this all up in another layer of brownish colored silk that she will form into a sac with her legs. She is capable of laying up to 4 egg sacs containing up to 1,000 eggs within each sac. She usually places the sacs at the center of the web where she hangs out. This allows her to guard the egg sac until she dies, which usually happens when the first frost arrives. The egg sacs overwinter and hatch in the spring. The spiderlings will "balloon" to new locations and build their own webs. Many of them will be predated on by other spiders, insects, birds, frogs and other creatures fond of tiny spiders. So even though a single female may produce up to 4,000 eggs, only a small percentage of them will survive to adulthood.
Humans have little to fear from these strikingly beautiful spiders. They are not known to be harmful to humans. Bites are only likely to occur when you grab one or mishandle one. A bite is reported to be no worse than a bee sting. If you have allergies to bee venom or other insect venom, then a bite from one could be medically significant and you should seek help.
Garden spiders are extremely beneficial to gardeners and farmers alike. They feed on harmful insects like crickets, moths and grasshoppers, controlling their populations and protecting your plants and crops from the voracious feeding habits of those insects. Not to mention watching them construct their web each morning is a fascinating thing to see. They should be welcomed to live in your garden and appreciated for the help they are providing, free of charge.