Very briefly:

• Intermediate range missiles have very short flight times. This makes them ideal for "decapitation" attacks if they can be based near a capital: if your time to arrival is, say, 5 minutes, then your time to respond is measured as even less than that. Can you get your leadership to a safe place, much less send out a retaliatory order, in that amount of time? Maybe not, esp. if your leaders are old or sick or drunk (an issue with three of the Soviet leaders in succession: Brezhnev, Andropov, Chernenko). This threat made the USSR very uncomfortable, because the US was basing weapons in Western Europe that could potentially threaten Moscow with only a minute or two of warning time. This in turn made the USSR adopt potentially dangerous, "hair-trigger" and "fail-deadly" policies (like the infamous "Dead Hand" system, which allowed a nuclear response order to be more or less automatic) that were meant to allow them to guarantee retaliation in a crisis, and that could easily lead to misunderstanding and accidental nuclear war (see: the Stanislav Petrov incident). So they were perceive as particularly destabilizing by the Soviets, and the habit of the US to put IRBMs into NATO allies near Soviet borders had long incurred Soviet ire (see: the Jupiters that Eisenhower agreed to put in Turkey, which were part of the provocation that led the Soviets to try to put IRBMs in Cuba, to disastrous effect). The deployment of fast, highly-accurate Pershing IIs to Europe in 1983 in particular made the Soviets very uncomfortable. So this was a class of weapons the Soviets would be very happy to not have to deal with, and the US had begun to appreciate how destabilizing and dangerous they might be.

• From the US/NATO perspective, banning intermediate-range weapons would also put the Soviets at a disadvantage with regards to Western Europe, since the bulk of the Soviet rocket forces fell into this category an the Soviets removed far more systems than the US/NATO did. So it is sort of win-win: Soviets lost a clear decapitation threat, Western Europe lost a major existential threat. Ultimately the Soviets removed more systems, but got more strategic stability as a consequence. A true "deal," one might say. This is not to say it was not controversial; the NATO powers in particular were not entirely thrilled with it.

• Separately, intermediate forces can be "use it or lose it" weapons (they are going to be hit first in a conflict, both because of their range and because of their relative fragility compared to an ICBM), which also encourages early use. (And if you want to ask, "how do you target mobile intermediate weapons?" — the US approach was, "nuke the entire forest they might be in," which leads to rather ghastly targeting consequences.)

• The INF was never intended (and was not) to be the final arms control treaty. But it was one that could be readily negotiated by both sides as a short of good faith. Longer-range forces do not pose the same "time-compression" effects as intermediate range forces. Very short-range forces (e.g. tactical weapons) lack the decapitation potential, and the US/NATO was, at the time, heavily dependent on them for deterring a large conventional attack from the Warsaw Pact (in the post-Cold War, this threat dissipated, and US/NATO decommissioned most of its tactical weapons; today, Russia is heavily reliant on tactical weapons to oppose the conventional threat posed by the US, which is an interesting inversion). Submarine-based weapons do have decapitation issues, but were so relied upon by both sides as guaranteed "second-strike" weapons that there was really no hope of negotiating them away. One could imagine negotiating away ICBMs (they are arguably unnecessary if you have SLBMs, and cause many strategic problems as they are "use it or lose it" weapons); it has been proposed many times but never implemented for various diplomatic and strategic reasons.

• INF was one of several arms control treaties and should not be viewed in isolation. It represented a major step forward in being able to remove an entire class of weapons. Other treaties targeted specific numbers of warhead, like SALT and START. INF was not meant to resolve all possible nuclear issues (and some of the modern-day objections, like "but it didn't include China," are kind of non-sequiturs — it was a bilateral treaty, these things exist).