Kevin de Bruyne is an excellent player. That's not just a couple of pre-season friendlies against weak opposition talking, either. He was superb for Genk for years and he was the top man at Werder Bremen last season. Borussia Dortmund wanted him as a replacement for the departing Mario Goetze. Everyone loves Kevin de Bruyne, and he can bring a lot to Chelsea Football Club.
But his play, while important, is almost a secondary consideration compared to what his presence actually means. The Blues were reliant, to a frankly ludicrous degree, on the play of Juan Mata, Eden Hazard and Oscar last season (although, as David points out, the Mata/Hazard/Victor Moses combination did very well too). Two out of those three ended up being worn into the ground. Mata's been overworked for his entire Chelsea career, while Oscar has played something like 100 football matches in the past 400 days.
Both of them need far more rest than they've been given to date. And that's where de Bruyne comes in. Unlike a Moses or an Andre Schurrle, neither of whom suffer from an overabundance of playmaking ability, de Bruyne can imitate the playing style of the trio, and that means he can realistically step in for any of them on a fairly regular basis.
This is crucial for keeping the heart of the team healthy, rested and effective. Having four options to choose from when Chelsea play their standard 'death by attacking midfielders' game is much better than having three must-pick players whom you select in every single match. De Bruyne's got the talent to become a starter should someone falter, but all four of them can impact the team in a tremendously positive way. Cycling through them will mean that the Blues can collectively play at a higher level for longer.
That's a very big deal.