What are practical applications of weak linking?

One use of weak linking is implementing the replaceable functions in the C++ standard. Namely:

void *operator new(std::size_t);
void *operator new(std::size_t, std::nothrow_t const &) noexcept;
void *operator new[](std::size_t);
void *operator new[](std::size_t, const std::nothrow_t&) noexcept;
void operator delete(void *) noexcept;
void operator delete(void *, std::nothrow_t const &) noexcept;
void operator delete[](void *) noexcept;
void operator delete[](void *, std::nothrow_t const &) noexcept;

These are functions which must be provided by the implementation, but if a program implements them then the program’s implementation replaces or overrides the implementation’s version. This is easily implemented via weak linkage.