If you you want to change a public method and can find all the type's usages, an IDE or search engine can tell you which call sites will break. (Or just compile everything and see what happens.)

If you can't find all the type's usages, you're screwed anyway because any change that would break an interface will also break a call site that calls a method directly, without using an interface. So having all the interfaces declared right there doesn't help that much.

From a large scale application developer point of view it matters a lot.

In code bases developed by 50+ developers across multiple sites, it is important to be able to look to a struct definition and be aware what interfaces in the code base are supported.