How important were SALT 1 and SALT II to the limitation of nuclear weapons in the period from 1970-1989?
If one looks strictly at the number of nuclear weapons that existed or were produced after the agreement was reached (even though the US did not sign the agreement after the Soviet invasion of Afghanistan) it does not present a clear picture of their effect on the production or deployment of nuclear weapons in the time period.
The two agreements served rather effectively as a means for the two superpowers to negotiate the deployment and make up of their nuclear forces. This opportunity to communicate about particular concerns or desires was important in setting the stage for future nuclear forces treaties such as the START treaties as well as providing some limits and reducing some of the paranoia surrounding the production of newer and more destructive or more accurate nuclear delivery systems.
The SALT agreements forced both the Soviets and the US to be somewhat more transparent about some of the weapons systems in use or in production including the newer versions of submarine launched ballistic missiles or aircraft like the Tu-22 Backfire, a new supersonic bomber produced by the Soviet Union.
The agreements certainly did not reduce the possibility of mutually assured destruction as the stockpiles of weapons may have remained relatively static, the accuracy and effectiveness continued to increase during this time. The agreements did appear to have something of a calming effect however as they did provide that vehicle for communication and discussion over weapons systems that were of particular concern to each nation.