How important were SALT I and SALT II to the limitation of nuclear weapons in the period from 1970 to 1989?Between the USA and the USSR
In this case, you can look at the intent of the treaties as compared to the reflection of the times when they were negotiated. Consider, for example, that SALT I was negotiated during Detente under Richard Nixon, at a time when both superpowers were seeking to scale back both their Cold War commitments and to reduce nuclear tensions. So the treaty itself, which froze the number of nuclear warheads held by each side, was an important step in arms control, but only took place because of the changing international political climate in the 1970s.
For SALT II, keep in mind this treaty was negotiated by Jimmy Carter in 1979, not by Ronald Reagan in 1989. So I'm not sure if you mean SALT II or the START I and II treaties under Bush Sr. in 1991 and 1993.
With the START treaty, the same is true of it being a reflection of a thaw in relations between the two superpowers, as well as a realization that the 27,000 nuclear weapons in their two stockpiles were expensive, unnecessary and unsustainable.
The two SALT treaties were the major treaties signed by the US and the USSR to limit nuclear weapons during this time. So, in that way, the two treaties were clearly very important to the limitation of nuclear weapons during this time.
However, the treaties were clearly not particularly successful in limiting the number of weapons each side had. For example, the SALT I treaty did not limit the number of warheads that each side had -- just the number of missiles. So the two sides raced to try to create devices called MIRVs that could have many different warheads, each aimed at a different target, on just one missile.
So the SALT treaties were important, but they clearly did not in any way end the arms race or significantly reduce the number of nuclear weapons in existence.