What all do you use R for?

I have just joined a lab for a PhD program (yay! woo! hurray! etc.)
Many people in my lab use R for various things and they suggested I should start learning it too.

However, when I mentioned about learning R when discussing a timeline of the next 3-4 months with my PI, he "warned" me to not use R for making simple graphs, there are other tools for that.

So, my question is what do YOU use R for, for which you wouldn't be able to use MS Excel or any other tool?