It is not additional software - it is simply another module of Avast Antivirus, not a new or unrelated program.
As drake127 said, new executable files appear (and disappear) in Avast installation folder daily, during virus definition updates - and sometimes even at other occasions, if needed. If the file is an "EXE" (which you are probably calling a program) or a DLL (a library), it doesn't matter, it's just a technical decision whether to make a piece of code an EXE or DLL (or something else, like a driver, or even just a "data file"). Btw, the difference between an EXE and DLL file is exactly one bit somewhere in the beginning of the file.
When I look at the installation folder of Avast Free, there's about 90 DLLs and 25 executables there - yet it's not an installation of 25 programs, it's just one - Avast Antivirus.
The discussed functionality (and much more) is already present in the "main" Avast modules, and has been there for years. It has only been extracted to (or possibly duplicated in) a standalone module in attempt to make the product more robust/reliable (and subsequently to provide a better protection with less compatibility issues).