It's an interesting question, but it kinda begs another one. And I'm only saying this because I happened to hear some former general on Fox News giving his take late last week (and quite honestly, I don't remember who he is, etc---I just know he's more qualified than I am to talk about military strategy) and he was kind of talking about it and posing the opposite question that you did. Basically he said "If Russia just wanted the eastern regions and a land bridge...having a column approach the most heavily fortified area in the country (Kyiv and the surrounding towns) was exceedingly stupid. There's no point putting all those assets at risk if you aren't trying to take that city. He (Putin) could have used those assets to completely shut the eastern part of the country off from the rest of it and marched around to the south to easily create his land bridge. The Ukrainians wouldn't have been able to stop it, and likely wouldn't have even tried. They didn't have the assets. Instead the Russians have stretched their logistical capabilities to the breaking point by running a massive column of equipment and personnel directly at the capital, and then spread out to surround it in what's seemingly a bit of siege warfare. Unless you're trying to topple the capital...why?"
I guess the answer to "Why does he want the capital?" COULD be that he believes he needs to topple their entire governmental structure in order to set up a puppet state. Maybe Putin believes (again, who knows...but maybe) that Zalenskiy leaving isn't even enough. He needs to rip it to the studs in order to get his people in there.
The other question is harder to answer imo: Why would he use so many assets for something he doesn't even want?