Multiple production rules for expressions of the same type?
In one word - ambiguity.

Sometime you don't want your grammar to be ambiugous.

For example, for parsing. Ambigous grammar can't have a $LR(K)$ grammar (you can think of ambiguity as non determinism). $LR(K)$ parser handle determinstic grammars.

Example for the ambiguity of your grammar:





The sentence a * b + c can be derived in two different ways.