I used to think that arrow boards were rather unnecessary. That was until I was coming home last weekend only to find that the exit to my house was closed for construction. Luckily, there were several arrow message boards set up prior to where I was supposed to get off.
Each arrow board had a snippet of information that told drivers where roads were closed and which alternate routes were optimal. This advanced warning allowed me to get off several exits early and take a shortcut that saved me at least twenty minutes. Although I still think that arrow boards are often overused, I now see their pragmatic qualities.