An experienced florist doesn't usually require a gridded vase, but it takes only a short time to quickly grid a vase instead of creating the grid with the greenery.
A greenery grid is often taught as the method for securing your flowers in the face. My biggest objection with this is that I like my water to be crystal clear when finished - not contaminated with bits of greenery being forced below the water line.
A clear grid made with narrow clear tape is a quick way to START designing with the flowers first - and inserting your greenery last.
You need to be sure your water is throughly mixed with flower food and your sides of the vase completely dry before gridding your vase. Don't worry about overlapping - the lines are virtually invisible after the flowers have had greenery added.