Why does the C++ linker require the library files during a build, even though I am dynamically linking?

I have a C++ executable and I'm dynamically linking against several libraries (Boost, Xerces-c and custom libs).

I understand why I would require the .lib/.a files if I choose to statically link against these libraries (relevant SO question here). However, why do I need to provide the corresponding .lib/.so library files when linking my executable if I'm dynamically linking against these external libraries?