Because he would own 17 Slams (at least), a Career Slam, and the most astounding thing would be: He would own 5 Grand Slams (at least) at 3 different Grand Slam tournaments. Federer held this record at 3 times at 3 Slams. He holds it even now at 4-4-4. But to make it 5 would be an unreal, astonishing feat of sustained excellence and endurance at the highest level the sort of which the sport has never seen before or likely will never see again. Even Sampras only managed 2 Australian Opens along with his 7 Wimbledons and 5 US Opens. If Federer can bag another Australian, it would elevate him to another level again altogether, as I do not believe anyone would in future be able to win 5 slams at 3 Slam events, not even Nadal. I am sure Roger is well aware of what this achievement would mean to his own legacy.