Why is -L needed when -rpath is used?

Even dynamic libraries required a degree of static linkage; the linker needs to know what symbols should be supplied by the dynamic library. The key difference is that the dynamic library provides the definition at runtime, whilst with fully static library provides the definition at link time.

For this reason, -L is needed to specify where the file to link against is, just as -l specifies the specific library. The . indicates the current directory.

-rpath comes into play at runtime, when the application tries to load the dynamic library. It informs the program of an additional location to search in when trying to load a dynamic library.

The reason -L/usr/lib doesn't need to be specified is because the linker is looking there by default (as this is a very common place to put libraries).

A clarification of OMGtechy's answer.

If the linker does not check which symbols are provided by a library, it can never tell you if any symbols are missing at compile time. They might be in one of the libraries loaded at run-time. You could never know. There is no connection at compile time between the header files of a library and the .so file.