Only beneficial thing for Russia is the Crimea defense SAMs used against ground targets in Nikolaev and Odessa are hard to shoot down so they usually get past Ukrainian air defenses. But it leaves Crimea more and more at risk.
The S-400 has a surface-to-surface mode (link, Link. The early Nike Hercules and Houthi rebel modified S-75 Dvina/SA-2 Guideline had surface-to-surface mode. Using SAMS this way is not new if there is nothing else to use and the means to modify the missiles are available.
I am guessing that for the S-300/400 in surface-to-surface mode uses either inertial guidance, radar guided to vertical drop, or combination. The S-400 can be used on exposed troops or unarmored vehicles. Considering all the all the field and rocket artillery Russia has, the only reason to use SAMS this way on Odessa and Nikolaev is to target the civilan population (Like the pro-Russia people say Ukraine is, because Russia would never do that).