What are fuses used for?


I have, I’m sure to many, a very dumb question. I keep reading about fuses.  I have a fairly sophisticated system and have never come across the need for a fuse.
Can someone tell me where these fuses go and what they’re used for?
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“Contrary to popular misconception by the ill-informed fuses (like all wire) have NO effect upon sound quality. To think otherwise is to believe in magic!”

Here we go again.....hope this thread doesn’t turn into heated debate on the effectiveness of aftermarket vs. stock fuses. 
If you have the manuals for your equipment it shows you where to replace a fuse. If you don't have the manuals you can probably download them from the manufacturer. 
Look on the back panel. Near the AC cord/outlet you will see a round (usually) screw-type cap. Below it will be a designation indicating the size of the fuse required. Unscrew the cap and pull it out. The fuse is held at one end by the cap. It is a press fit. Pull on the fuse to remove it. Hold the fuse up to a light and observe the very thin wire within. If it is intact the fuse is good. If the wire is broken the fuse needs replacement. Replace only with a fuse of the same rating on the back panel.