Well like you said, having extra space for electricity generation for future weapons is probably a big part of it. I doubt the initial 055 variant will have IEPS, but later variants almost certainly will, and will thus have the capacity to be refitted or even built with rail guns and lasers.
More generally, having extra space means more growth potential overall. That is a big problem cited with the flight III burke, namely that they are increasingly squeezing too many things on a hull that's not getting any bigger. PLAN reached its limit with the 052 hull on the 052D, so they're taking the long view with a hull that is now much bigger.
Of course a larger hull is also driven for a need in capability as well. PLAN's VLS is quite large, so they need a reasonably large ship to be able to carry anywhere upwards of ~100 such cells. PLAN may also be interested in fielding a larger, more capable ASW helicopter (Z-18FQ) which also necessitates a bigger flight deck, hangar. Internals are also likely to be more spacious and automated. Crew size of such a vessel will likely not exceed the crew for an older generation DDG, yet there will likely be better accommodation for crew as well. A ship of this size will also likely have an enhanced CIC and room for a flag officer and their staff, to fit the profile as a command ship. PLAN also seems quite ambitious in giving it a very advanced radar capability in the form of multiple phased array radars that are all more specialized in their roles than one another, suiting its position as an air (and potentially space) defense ship first and foremost
Combine all those demands, with current and near term future technology, and it's hard not to get a ship of around 055's projected displacement.
So it is definitely capability needs that is driving size, rather than the other way around.
Some USN commentators have also said that the Flight III burke should have been replaced by a 12,000 ton new hull design as well. I also suspect the USN's future main surface combatant programme will lead to a ship of similar displacement to 055.
Russia is also supposedly developing a new DDG of similar dimensions and displacement.
Also, it isn't that unique for the PLAN to be confident enough to finally go out on their own and put out demands for themselves. They've reached a capability level where they can do it, and their industry is there, so why not? After all, the USN is still the most demanding customer with 60+ 9,000 ton destroyers in service, when for so many years the only other navies that had ships of that weight class were the Soviet/Russian navy and then later the JMSDF.
