I am not an expert on the Symbolic Toolbox, but this is what I have:
syms defines a number of symbols simultaneously. For example:
syms x y z a b c
defines six symbols. Now, you can use them the way you want:
f = a*x + b*y^2 + c*z/4;
As a result, f will be a symbol again [and you will not need to define it using syms].
Personally, I find the symbolic toolbox useful when I need to do algebraic multiplications [for example, for the multiplication of two polynomials]. Rather than doing it by hand, which is prone to error, It is much easier to define the two polynomials and then multiply them. I can then use the result the way I want in any sort of program.