The Chinese have always copied Soviet/Russian aircraft, and used similar combat philosophies in design and deployment. The Russians have had an ongoing string of gigantic aircraft designed for long-range, high-altitude defense and interception to protect their vast length of border. China is a large nation, but nowhere near as large as the Soviet Union, or even modern Russia. So why the need for a big, long-range defense fighter, possibly with very high altitude capabilities?
I think the primary potential adversary that China worries about is the U.S., possibly in combination with other western powers and some local nations that allies of the U.S. China's air force has some more modern aircraft, but they would mostly not fare well against a mixed force of F-22s and F-35s, backed up with a whole lot of earlier generation fighters. In addition, many Chinese airbases are within a hundred miles of the coast, and are quite vulnerable to cruise missile strikes. If China based most of their forces forward near the coast, the attrition rate would be very high and very rapid.
The Chinese are not stupid, and I think they have a different strategy in mind. If they created bands of dense and sophisticated, redundant SAM defenses, that would be tough for an attacking force to deal with. SEAD planes and drones would go for active RADARs, which could be real or decoys. The Chinese would activate other RADARs and launch SAMs only when attacking planes were overhead. But SAMs are not enough by themselves. Airbases farther inland could host conventional aircraft to harry attacking aircraft with long range AAMs as they are trying to deal with the SAMs. Other conventional aircraft would be tasked with using their look down/shoot down RADARs to intercept air- and sea-launched cruise missiles. Far inland in well-protected airbases, J-20s could be operated in relative safety, behind the multiple bands of defenses. They would have the range to fly defensive missions from their safe deep inland bases out beyond China's coast, and attack the most vital assets; as has been said, the AWACs, P-8s hunting Chinese subs and tanker aircraft. If those assets can be sufficiently degraded, attacking into Chinese territory becomes much more difficult. I would be looking for word of development of a Chinese copy of or long-range AAM similar to the Russian Vympel R-33 missile. A longer-range version of the Pl-21 AAM might fit the bill.
In addition, in the tradition of high-altitude Soviet/Russian interceptors like the MiG-25 and -31, the J-20 coming out to meet an incoming attack force could be a formidable adversary. With its LO design, it would be difficult to spot. I would bet it also has an LPI RADAR. If, like the two aircraft mentioned, it carried large, long-range missiles, it could move in on an attacking American force, fire from 20-30k feet above the attacking force at a distance of over 100 miles, then turn and escape easily after disrupting the attack. Another flight of J-20s could come in a ways behind the first to break up the attacking aircraft again if they reformed after the first missile barrage. This would be a very difficult defense tactic to overcome.
This is all pure speculation, of course, but it seems to be a viable strategy. One other factor that makes this even more likely is that China was going to purchase some MiG-31s from Russia, as they were reportedly very impressed with their capabilities. It would not surprise me at all to see similar capabilities designed into the J-20.
