Discord isn't integrated with rsi.com ... Spectrum is. Whether or not that's important to you is another matter, of course.
Just that. The idea behind Spectrum is a good one: an integrated communications package that functions equally well from within the game as from a web browser or a mobile device. It's not exactly core technology for the game to function, but it's a good idea.
I suppose there might have been easier ways to achieve the same result, such as reaching an agreement with the Discord people and providing an in-game Discord client. But an advantage (to RSI/CIG) of the completely in-house approach is that CIG/RSI can oversee and moderate what is discussed.
I'm sad they haven't included voice communication yet. That's one of those "Soon(TM)" things. As Spectrum is in Alpha, it's presumably intended for testing, not for actual use.