The parent comment is complete nonsense. The motive for making low-yield weapons is the same as the motive for making high-yield ones. Deterrent. You can simply look this up in the DoD’s Nuclear Posture Review:
> Russia’s belief that limited nuclear first use, potentially including low-yield weapons, can provide such an advantage is based, in part, on Moscow’s perception that its greater number and variety of non-strategic nuclear systems provide a coercive advantage in crises and at lower levels of conflict. Recent Russian statements on this evolving nuclear weapons doctrine appear to lower the threshold for Moscow’s first-use of nuclear weapons. Russia demonstrates its perception of the advantage these systems provide through numerous exercises and statements. Correcting this mistaken Russian perception is a strategic imperative.
> To address these types of challenges and preserve deterrence stability, the United States will enhance the flexibility and range of its tailored deterrence options. To be clear, this is not intended to, nor does it enable, “nuclear war-fighting.” Expanding flexible U.S. nuclear options now, to include low-yield options, is important for the preservation of credible deterrence against regional aggression. It will raise the nuclear threshold and help ensure that potential adversaries perceive no possible advantage in limited nuclear escalation, making nuclear employment less likely.
https://media.defense.gov/2018/Feb/02/2001872886/-1/-1/1/201...
To say the purpose is tactical use in war is franking a baseless conspiracy theory, relying on flawed logic that can easily be debunked.