Offset spatula's are ideal for spreading frosting and fillings and have become such a necessity in my life that I have two of each size- large and small. I couldn't image not having both sizes because they're necessary for different functions.
I use the larger spatula when I need to span a broad surface- like I did when I spread frosting for this cake.
The smaller spatula's petite size makes it perfect for anything that requires more attention to detail, like a cookie or bar. I used it here to gently spread frosting on the tops of the cookies.
Most kitchen stores will sell them- these are great from here and here. Have you ever used an offset spatula or do you own one?