What does the linker do and why is it necessary?

It combines object files, pulls in the C runtime and library functions, and resolves cross-references into one runnable executable.

* The linker merges your object with the C runtime and libc into one executable, resolving each external symbol reference to its definition. *

$ gcc -o p24 p24.pp.o
$ ls -lrt
# Original source
-rw-r--r-- 1 devel users   270 p24.c
# Preprocessed
-rw-r--r-- 1 devel users 48548 p24.pp.c
# Assembly
-rw-r--r-- 1 devel users  1592 p24.pp.s
# Object file
-rw-r--r-- 1 devel users  2072 p24.pp.o
# Executable!
-rwxr-xr-x 1 devel users 15648 p24

Why linking is needed:

• Object file only contains code for main()
• Missing: C runtime startup code, library functions (printf, etc.)
• Linker combines everything into final executable

Go deeper:

• Linker (computing) (Wikipedia) — combining object modules, resolving external symbols, pulling in libraries, and relocation to produce the final executable.