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> Why source being available makes difference?

Because the library is a spec, they don't have to be compatible at a binary level. This is obvious on a hardware architecture level (a 16-bit processor vs a 64-bit processor) but it's true even on the same hardware under the same OS (the different representations of std::string being a famous example). But they are all compatible at the API level -- which in C++ is the source level.

So this is why I mention compatibility at the source code level.

> What is so beneficial in having different implementation of same functionality? We can ignore cases of optimized per platform implementations because std was not made for that.

The point of having different implementations is that not every program has the same needs. The idea of a standard library is that it has a bunch of commonly-used functionality that you can just use so you can concentrate on your own program. You don't have to roll your own string class -- just use the standard one. You may later find you have special needs and need to roll your own, more restrictive string that does just what you want, but in most cases people won't have to. (You can argue whether the C++ standard library accomplishes this or not, but that's a separate matter).

A good example of this is std::map which was overspecified, and thus almost never what you want. If the specification had been looser than different implementations could have chosen different solutions, and even borrowed from each other.

And per-platform optimization is exactly part of std's requirements. Different implementations for the 16-bit and 64-bit cases is an easy to use example. The compilers output a lot of intrinsics to take advantage of CPU capabilities (a common example is memcpy, but there are many). Just try to read the source code for libc++ -- it's hard to read when you aren't familiar with it because it's 1 - full of corner cases so that it works with any code that uses it but also 2 - it's full of target-specific optimizations and special cases.

> None of platforms that make sense to support now were available back when current ABI was set in stone.

Well this is true, but since it's a spec it remains source-code compatible.

I don't know if your use of "ABI" was a typo for "API" or if you really meant "ABI". The APIs are set in stone because of back compatibility (like the notorious std::map example I call out above) but use of ABI, when used by the committee, refers to the de facto binary layout of code that's already been compiled. There are few platform ABI specifications for C++; they are mainly for C, with sometimes some Ada or FORTRAN calling convention stuff. Rarely do platforms say anything about C++, and when they do they don't specify much. There is also a little ABI requirement in C++ (e.g. address of a derived class must also be the address of its own most fundamental base class) but that's for something that is reflected at the source code level.