russia seen eastern ukraine as the safety barrier to keep nato away from russia's borders.
its why russia wanted to take that eastern region and declare it an independent state (no mans land) to keep that buffer space to keep nato/EU at arms length so that russia does not need to defend it endlessly forever
That's one of the smaller reasons since the annexed regions are also considered "Russian borders" now (95% of them voted for Putin in the recent election in Russia).
The major reason is for Russia to
dominate the Black Sea considering that for such a massive country Russia had a very little sea shore and control in the Black Sea before 2014. Also if you look at the annexed parts on the map, these regions are more of South and South East rather than being Eastern Ukraine. Russia still shares a lot of border with other Ukrainian provinces in East and North Eastern Ukraine, I marked that huge border with red in the picture below and the new borders with the "buffer space" in green. You can clearly see the difference this way.
By taking Crimea they choked Ukraine in Sea of Azov and increased their dominance in the Black Sea. By taking more land in South and South East (basically everything below Dnipero River) they cut 90% of Ukraine's access to the Black Sea and increased their own dominance.
These regions also included major industries, infrastructures, farms and water sources of Ukraine that now belong to Russia.
In Russia they can write whatever they want into their Constitution about their territorial conquests of other states. Not a single civilized state takes this seriously and will not recognize it.
Moreover, Russia no longer dominates the Black Sea. The remnants of its Black Sea Fleet are hiding away from Ukraine's maritime surface drones and its Neptune anti-ship missiles.
So, only recently, on March 24, the Ukrainian Armed Forces attacked the port of Sevastopol and damaged the large landing ships Yamal and Azov and Konstantin Olshansky, and also successfully defeated the reconnaissance ship Ivan Khurs. Russia has moved most of its ships either to Novorossiysk or to its other ports away from the Crimean peninsula. And everything that is located near the Crimean peninsula is being actively destroyed by the Ukrainian Armed Forces.
Russia will not be able to hold the south of Ukraine for long. First of all, because ammunition and everything else have to be delivered 700-800 kilometers and this entire route is under fire from the Ukrainian Armed Forces. It is difficult for Ukraine to maintain control of the east; there are direct supplies for the front from Russian territory. And the Black Sea and Azov coasts will soon be liberated by Ukraine.