Are those external fuel tanks? Is it just testing/demo or they need to manuever the J-20s to somewhere far away?
Yes. We've had pictures of J-20s with EFTs from months ago, this isn't a new thing. They're probably testing them, or ferrying them somewhere with the EFTs as the practical way of doing it.
As for why J-20 could use EFTs, well what EFTs do is they provide extra range/endurance for an aircraft, but on a stealth aircraft they obviously compromise stealth.
For J-20, some people have suggested J-20s will carry EFTs during combat operations to extend their range or endurance, but I think this is only half accurate.
(following is x-post from secretprojects):
I wouldn't feel comfortable having a stealth fighter fly beyond one's IADS defended borders with EFTs during war time. I think during a conflict, J-20s would not fly outside of China's borders with EFTs.
I believe the primary use of EFTs would be to support rapid redeployment from bases within China's borders during conflict, where they take off from one base (say in central or western China) loaded with EFTs and internally loaded with AAMs or other ordnance for their mission, and then as they near the theatre of operations (say, the airspace over the East China Sea beyond China's IADS borders) they would eject their EFTs+wet hardpoints and proceed with their mission in a clean configuration. After their mission, they would land at a base near their theatre of operations where support crew and logistics would have been ferried and/or pre-placed to support sustained operations from that airbase.