Bottom up preference. Russian pilots simply prefer using the Su-34 over the Su-30 on bombing tasks, even though Su-30s are capable of dropping FABs, and they do so once in a while to flex their muscles. But the overwhelming preference for the Su-34 lies in the side by side seating arrangement which appears to have enormous benefits in morale and cockpit productivity.
I doubt navigator has any use beyond taking videos for GNSS bombing. There are many Su-34 specific problems - for one, as aircraft was born after the fall, it's way less capable than it should've been(in many ways su-24m is better just because many core things in su-34 weren't implemented due to lack of funding).
Main reason, realistically, is institutional inertia carrying on from the 1980s. There was to be a bomber because there was a bomber before.
During SMO specifically, - much surdier airframe (for years of continuous bombing and daily sorties), and ironically armor (su-34 crew losses are very low). There are also new uses for navigator which emerged /now/, which do indeed benefit from wide cockpit.
The same situation is happening over the Ka-52M over the Mi-28NM helicopters. The side by side seating is more popular but the MoD also needs to keep the Mi helicopter production lines busy.
This one is very interesting. Side by side seating is normally outright bad for combat helicopter as we know it - it adds a lot of weight(especially if armored) and vulnerable frontal area; there are no redeeming sides from this point of view.
Ukraine wasn't easy on combat helicopter, however. And if we use it not as dedicated AT helicopter or even combat helicopter(both are essentially supremely fast and amphibious land vehicles), but rather than as tactical VTOL fighter-bomber, ka-52, with it's convenient cockpit and far better radar turns from a weirdo into harbinger of future.