This sounds like a duplicate version of What is the point of internal linkage in C++ and probably is. There was only one post with some code that didn't look like a practical example. C++ISO draft says:
When a name has internal linkage, the entity it denotes can be referred to by names from other scopes in the same translation unit.
It looks a good punctual definition for me, but I couldn't find any reasonable application of that, something like: "look this code, the internal linkage here makes a great difference to implement it". Furthermore based on the definition provided above,it looks that global variables fulfils the internal linkage duty. Could you provide some examples?
Internal linkage is a practical way to comply with the One Definition Rule.
One might find the need to define functions or objects with plain names, like sum, or total, or collection, or any one of other common terms, more than once. In different translation units they might serve different purposes, specific purposes that are particular to that, particular, translation unit.
If only external linkage existed you'd have to make sure that the same name will not be repeated in different translation units, i.e. little_sum, big_sum, red_sum, etc... At some point this will get real old, real fast.
Internal linkage solves this problem. And unnamed namespaces effectively results in internal linkage for entire classes and templates. With an unnamed namespace: if a translation unit has a need for its own private little template, with a practical name of trampoline it can go ahead and use it, safely, without worrying about violating the ODR.