That brace doesn't have a lot of torsional strength (can't take a lot of twisting or sideways pressure), but it has a lot of linear strength (i.e. pulling it like taffy won't break it). That keeps the long sides of your tank from bowing out, and relieves a lot of pressure on the end seams. It is necessary. You can either order a replacement top frame from your LFS, or build a DIY brace by siliconing a piece of 3-4" wide glass where the brace used to be. Meanwhile, you would be well advised to lower the water level in your tank substantially (30-50%) until you can get that support replaced.
Look at it from this perspective - with all the cost cutting that goes on in every industry, do you really think that the tank maker would go to the expense of adding that brace if it wasn't necessary?