Why do I need a straight edge on my lawn?
I suggest you take a good look at a property with a straight edge, and then take a good look at a property without a straight edge. Huge difference! Even when the lawn is mowed regularly and the hedges all pruned and the flowers and plants looking beautiful, the edge still makes a huge difference in the asthetics of your property. Around here many of the grasses and weeds are crawlers; this makes an un-edged lawn even worse with grasses crawling out over the sidewalks.
Think of it like a coloring page. A page colored all over inside and outside of the lines looks messy and unappealing. A coloring page deliberately colored inside the lines appears much more pleasing to the eye; it gives the eye boundaries and allows you to see the flow of the landscape. String trimmers only get some of the work done. It is important to hard edge on occasion as well.
Shadow edge is a 1-2 inch lip that separates the lawn from the bed. It is called a show edge because of the small shadows it casts to the lower surface of the bed. I prefer the shadow edge because it is easy to maintain and looks excellent as a beautiful crisp edge to your lawn. The trough created also helps keep the grass from spreading into the bed.
Metal edging is great for gravel bed or pathways, because edging and string trimming around gravel is difficult and dangerous as it kicks up chucks toward windows, and faces. Metal edging separate the two nicely for easier string trimming on the grass side. Metal edging is unnecessary for mulch borders, a shadow edge gets the job done so there is no need for extra cost or work.
Other Curbing (Brick, Concrete, Stone or Rocks)
Rock or concrete borders are not good borders for lawn. They make it a nuisance to maintain as the grass grows all around, in between and in the cracks. Edging and string trimming is difficult and most of the time you still have to get on your hands a knees to weed it by hand. These types of edging are great for borders to gardens as long as no lawn is near them.