Reaches 2 mK[ref]. https://youtu.be/upw9nkjawdy?t=487 from Video "Building a quantum computer with superconducting qubits by Daniel Sank (2019)" mentions that 15 mK are widely available.
Used for example in some times of quantum computers, notably superconducting quantum computers. As mentioned at: https://youtu.be/uPw9nkJAwDY?t=487, in that case we need to go so low to reduce thermal noise.