I note they say
"Note that rtools40 is only needed build R packages with C/C++/Fortran code from source. By default, R for Windows installs the precompiled “binary packages” from CRAN, for which you do not need rtools!"
I am pretty sure I have R for windows (everything we do uses that). But some R packages (I ran into one yesterday) still will not work without Rtools at least some functions will not. And I get warnings every time I import a new package that I need it.