How do import libraries work and why doesn't MinGW need them?

It is possible to link to DLL without an import library as MinGW clearly demonstrates. Hence the question is why MSVC decided to omit this feature.

The reasons are primarily historic.

Back then in 1983 when Windows came around and DLLs were designed there were many toolchains (compilers, linkers) from different vendors. Going out asking the vendors to implement support for linking "DLLs" for a minority OS was clearly not an option.

So they made a decision to write a tool to generate a library everyone and their dog could link against even if a linker had absolutely no idea about DLLs.

Besides, import libraries offer some features that were vital 3 decades ago but are next to obsolete now. First is the ability to import a symbol by ordinal — i.e. DLL has an option to offer no names at all only a list of addresses; the ordinal is an index in this list. Made sence when the amount of RAM was severely limited.

Second is the support for different name mangling schemes. Even in C there is a name mangling scheme, for instance FooBar may become _FooBar@4 (it depends on the platform and the calling convention). It made perfect sense for a DLL to export "FooBar" on every supported platform for consistency (and it makes the life of GetProcAddress() user easier). Import library implements the mapping from _FooBar@4 to FooBar.

This is based on the blog (1, 2) of Raimond Chen, the man who was involved in Windows development from the very beginning.