I can't speak for him, however as a programmer speaking to other programmers, I find Slack is pretty nice primarily for these reasons:
Pasting code snippets, preserving formatting and getting syntax highlighting.
Pasting screenshots, usually of code, error logs, or graphical bugs.
Inlining code with `def func_name()` or whatever, making things much more legible.
Easily sending files.
Basically all code manipulation at their root.
For actual normal communication, IRC works just as well for me.
