What are the two main reasons for using linkers?

Modularity (split a program into separately-compiled files and reusable libraries) and efficiency (recompile/relink only what changed, and include only the library code you actually use).

Modularity:

• Programs can be split into smaller source files
• Teams can work on different files independently
• Libraries provide reusable common functions (math library, libc)

Efficiency:

• Time: Separate compilation - only recompile changed files, then relink
• Space: Libraries aggregate common functions, but executables only include what they actually use

Example:

# Only recompile the changed file
# Changed file
$ gcc -c swap.c
# Relink
$ gcc -o prog main.o swap.o
# No need to recompile main.c!

Tip: This is why large projects use build systems like Make - they track dependencies and only rebuild what changed.

Go deeper:

• Linker (computing) (Wikipedia) — overview of separate compilation, symbol resolution and relocation.
• Ian Lance Taylor — Linkers (full 20-part series, indexed) — a linker author's definitive tour of how and why linkers work.