What does changing Joules in TC mode affect?

I've been vaping on my subox mini for a while now. I like it very much. My friend just got a sigeli 150tc and I'm thinking about following suit and upgrading to that.

While using his mod it was in TC mode with some nickel could on it. Instead of changing watts the buttons raise and lowers the joules. Neither of us noticed any change in the vape experience wether it was at 50 joules or 80 joules.

What's the purpose of changing this number?

Thanks.