Ukraine has long sought after long-range ATACMS. Meanwhile, White House and military planners have carefully considered the risks associated with providing the weapon and they've decided this is the right time. If used properly, it can knock out Russian troops and logistics further back from the frontlines and turn around Ukraine's fortunes in the war.
Ukraine has already used more limited weapons to launch a series of terror attacks on Russian territory. Now that the US has provided Kyiv with long-range ATACMS, Russia can only expect these terror attacks to continue and escalate. Now, more Russian cities are in danger and Crimea is in range. Russia will have no choice but to respond, and it will have no impact on the mess that the US left Ukraine in.