Looking at physical map of Ukraine (below for reference) the river turns towards Russia and Donbass up to Dnipro and most of the river south of Zaphorizhzhya is already under Russian control.
In addition, it is very hard to launch attacks across the river because at the most critical point - the stretch south of Zaphorizhzhya - it is essentially an internal sea, with the river south of Zaphorizhzhya being the Kakhovka Reservoir which is 23 km across and 240 km long. Russia took control of the Kakhovka Dam on February 25, so the reservoir's water level is under Russian control.
After taking Donbass it is natural to pursue retreating UAF troops westwards who are likely to flee towards Dnipro and Zaphorizhzhya. At that point a cutoff in the wilds near the Samara River will block them and cut off those cities from reinforcement simultaneously. Taking Zaphorizhzhya means total control of the lower Dnieper. Taking Dnipro would mean that the entire southerneastern Ukraine is 100% secured, as it would also mean controlling the Samara River, another natural boundary.
The best case scenario for Russia is to push towards natural boundaries so that at each step, the Ukrainian forces can't take back their losses easily, whether that be in a counteroffensive in this war, or fighting another war in the future (if they survive).
