What are the philosophical grounds for Frege's notion of 'sense'?

To understand Frege’s notion of ‘sense’, it is imperative to grasp the problem it aims to address. It is a fundamental principle of logic that if two singular terms (a,b,c, etc.) are identical, then they must share all their properties. It follows from this that for any case where a=b, whatever is true of a must be also true of b. However, what of the sentences ‘Lois thinks that Superman can fly’ and ‘Lois thinks that Clark Kent can fly’? Even though we know (let’s assume) that Clark Kent=Superman, we still want to declare the first sentence true but the second sentence false. This stands quite clearly in contradiction our logical principle that we do not want to sacrifice. Frege here introduces his concept of ‘sense’. Whilst Superman and Clark Kent have the same reference i.e. those two distinct singular terms pick out the same object, they have different senses. Sense is the meaning of a term, and the sense picks out the thought or concept that the term expresses rather than the object. Now, we are able to explain that although the terms Superman and Clark Kent pick out the same object (they have the same referent), they do not pick out the same concept (they have different senses). For instance, Superman is ‘a man who wears capes’ whilst Clark Kent is ‘a man who wears glasses’. This allows us to maintain our logical principle whilst providing an explanation for these seemingly tricky counterexamples.