I could see a benefit to occupying a small amount of Russian territory, not a whole lot as to not risk compromising foreign support, but something, simply because the war is very unlikely to end with a total defeat of the Russian state, so a Ukrainian victory would mean Russia deciding that it was no longer worth it and leaving, or Russia getting pushed out of occupied lands and then the conflict settling into a frozen conflict. The former is probably more likely than the latter given attacking entrenched positions is difficult, so having some bargaining chip if Russia decides to end things makes it more likely that they pull out entirely rather than try to just stop with what they occupy, since freezing the borders as they are in such a case would mean giving up their own territory.