And in a way, everything is a CLI tool on most normal systems. Evince or Acroread or whatever you prefer to read PDFs is not "a CLI tool", but if I want to use LaTeX to create a document, I want to be able to do something like
$ xelatex myfile.tex
$ evince myfile.pdf &
I don't want to have to build my document, bring up my app launcher, click on the Evince icon, hit Ctrl-O, navigate to my pdf file, and double click it.