MacTraps is the binary needed for routines that require “glue” code or the “[Not in ROM]” traps from Inside Mac. You will need to include this as a project file in almost every project. It includes all the Mac OS and Toolbox managers as of the System 7 era.
MacHeaders is precompiled version of headers for only the most common Toolbox/OS managers, provided to save compile time for standard apps that use these most common functions, and preincluded in the standard prefix to save you from always having to #include <QuickDraw.h> in every single project. So it doesn’t include everything. If you want to use for example the Palette Manager, its headers aren’t part of MacHeaders so you will need to #include <Palettes.h>, but you still won't have to add any more libraries because MacTraps already has you covered.