I quite literally answered it in the same comment you're replying to:

Russia is working on wearing down Ukraine’s arms and forces in a war of attrition, spreading them thinner and thinner and advancing slowly and thoroughly.

War is not "take XYZ land and then you've got it," it's a battle between two powers. Even if your goal is to annex certain territory, in this case the four Oblasts that Russia has made clear it intends in taking, taking land outside of the four Oblasts weakens Ukraine's ability to launch counteroffensives and stage troops. It does not mean Russia will keep every bit of land they take. Further, it also gives Russia leverage in treaties.

This is incredibly simple and I'm astounded that you're making me repeat this in ever-simpler terms.