Does it really matter if you believe in the devil?
Well yes and no, I would say it helps to understand how humans, and the sin nature works out in peoples lives, so if one looks at Satan's slow decline from being the highest and brightest of creation to his fall, we know what to look out for, so we can guard ourselves against sin. Also we can see the world and man following Satan's decline so to speak.
I think perhaps he's a personification of the dark side of mankind, or one could say the worldly side of mankind.
That is pretty much how I view Satan, he is personification of sin, I think the bible uses him brilliantly as an example or pattern of sin.
In the gospel accounts Satan and the other fallen angels while spirit beings physically manifested themselves in humans, they could also talk to men, for example when the demon said to Jesus "my name is legion for we are many" it was real for we would also have heard him if we were there.
This does not happen today as all the angels that sinned are cast out and now bound. Peter for example records this fact for us about 20 years after the crucifixion.
In the non Christian world Satan and the other fallen angels are pretty much counted as not existing, as they currently don't wander the earth, obviously there is a difference from the gospel accounts where fallen angels were active and all would have believed in demons as a fact, as it's hard to ignore a physical reality.
Today Christians seem to want to give a place of power and prominence to Satan and demons I wonder if they even know it's a form of worship, he is not even here and yet he is still literally the god of the world for some.
Anyway that's another topic, I for one do not give Satan or the fallen angels any place of prominence or ability to affect me in anyway, I see any problems I have as a result of sin affecting me and my thinking, I do not want to follow Satan's footsteps at all, and if he can fall, then I am just as, if not more susceptible to give birth to sinful desires which in turn turns to rebelling against my creator.
So I drown myself in Gods Word, He is the best protection against our sinful nature, He Has set the pattern of living He wants for us all, which He knows is best for us, in turn this brings us true understanding, contentment and happiness.