Once you get past that, however, it is vastly different. Unlike WS, you control most of the factors which affect your chances of hitting your target. Range, movement, whether your opponent has cover all matter when it comes to figuring out how likely you are to hit.
Clever use of movement can deny cover and close (or add) range, but at the cost of the shot being more difficult*. Of course, it can also put your precious missile troops right where they want to be...at risk of being in close combat.
As a result, determining the best course of action to maximize your shooting relies on a good battle plan, clever maneuvering, and then your bs.
The side note is that many missile weapons only have S3. So even if you hit your target, you often will either see your shot bounce off the target harmlessly or even see it deflected by the most humble armor. This can be extremely frustrating.
Of course, at the other end of the spectrum are stronger weapons; S4 handguns, Bolt Throwers which allow no armor skill, etc.
The net result is shooting has a wide range of effectiveness and different weapons have to be used in different ways. It is one of the ironies of Warhammer that sometimes it is counter intuitive which missiles are best used against which targets. One example would be skinks.
Their short range, low S javelins, for example, require them to be exactly where they do not want to be; close to monsters like a Sphinx, for example, which can thunderstomp them to death in a flurry of high S auto hits. Yet the lack of armor on the Sphinx means their poison is very deadly to the Sphinx...and the close range, low S nature means they are much less valuable against the inexpensive skeletons.
So how important is a high BS? As with so many things in Warhammer, the answer is, "it depends". What is the role of that troop? How reliably can they deliver the shots when and where they are needed?
In 8th edition the amount of time a unit can reasonably expect to have viable targets and opportunity to fire is seldom going to be more than 2 or at most 3 turns per game. As a result, they need to maximize the damage they do in that short period of time.
At some point in the future I might write a piece on target selection, which matters hugely. But for now, I am going to stick with determining how important BS is.
On the one hand, it is less reliant on M than WS is since by definition it has a range greater than base to base contact.
However, it is more reliant on other factors which are more controllable than WS and one that isn't. The fact that most shooting is low S means the probability of doing a great deal of damage is generally limited. There are of course numerous exceptions;
Units that use poison, weapons giving S4 and/or armor piercing, units that can put out ridiculous numbers of shots.
But by and large, even the best missile troops are going to, at best, thin the numbers they or their close combat compatriots will face when the lines smash into each other.
It is important to use it well, and for some armies it is more important than others, but in 8th edition, BS is important but not the most vital.
* Not including the variety of weapons that ignore movement or other factors. And Tomb Kings.
