Why use precompiled headers (C/C++)?

It compiles a lot quicker. C++ compilation takes years without them. Try comparing some time in a large project!

In C/C++, the #include mechanism is a textual copy of the file specified into the current file. Headers include other headers (which include yet other headers), so when you do a #include, it could be adding tens of thousands of lines of C++ into each cpp file (or cxx, c, whatever), all of which need to be compiled each time. This can be a severe bottleneck for large projects.

Precompiled headers speed this up by compiling each header once, then including that compiled state into the cpp they are included in.