They're important for... Organising a track I guess I'd call it? Metal by and large is quite a chaotic and overwhelming genre and while the riffs and energy are the big appeal, on their own they would likely turn into a meaningless series of segments, hell it happens all the time already.

Plus some bands like to use good vocalists to their advantage. Anaal Nathrakh obviously like to utiilse the full range of their vocalist.

In my experience, vocals tend to be considered a little less important than drums and guitars in metal. each genre has its sort of defined vocal style and bands don't put much of an emphasis on trying to be particularly unique in that regard.

I, for one, find that vocals can make or break an album a lot more than any other instrument.

for the life of me, I can't imagine what kind of metal you've been listening to...

I've sampled a lot of the subgenres, but have listened the most to death, black, and thrash. All three of those genres have a very distinct style to each of their vocals that vocalists in each genre seem to intentionally emulate. Thrash has a harsh hardcore-influenced shout/scream, death obviously has death growls, black has the high-pitched shrieks. Hell, within DM alone, tech-death has a very distinct style of death growl (I've seen reviews on MA lambast tech-death bands for this so it's not just me), and BDM obviously has those weird gurgles/squeals.

This observation of mine has been influenced by metalheads who seem to absolutely ignore the vocals when listening to non-metal and then complain about the instrumentals being boring when the vocals are the point in many such genres. Post-hardcore is mostly chugged power chords and a few simple melodic hardcore riffs, really blowing my mind here. Almost like the point of pxc is the vocals and overly technical instruments would get in the way of that or something!

So yes, vocals can be very important. Dying Fetus, not necessarily, but Judas Priest yes.

I can't imagine Dying Fetus without John Gallagher's vocals =)

In sports, there is a concept known as "floor raising" and "ceiling raising." A "floor raiser" takes a very poor product (or team) and is able to turn it into something decent, but cannot be put on a decent product and elevate it to something great. A "ceiling raiser" is something that cannot make a poor product decent, but elevates a decent product to the next level. Great vocalists are a "ceiling raiser." A band can be extremely tight, have strong riffs and musical performances, but if the vocal talent isn't there, it can hold back the music. A great vocalist can't make shitty music good, but they can make good music even better.

To put it another way, quite a lot of metal makes good instrumental music in the way that certain genres don't really. When bands write songs, the vocals often take on a high degree of importance (say, 'The Number of the Beast' or 'Seek & Destroy'), but when they are more interested in a composition the vocals are less of a focus; for instance, I would definitely still enjoy 'Wake Up Dead' or 'Hangar 18' without the vocal sections. These are all examples of fairly melodic, classic metal, which tends to emphasize vocals at least equally with guitars and drums, but I think the same is true for extreme genres too.