Designing with fuses...when to use?
Fuses protect against fires.  If you have a non-current limited supply (like a connection to mains power or a large battery) and you develop a short on your PCB, it's possible to ignite something.  

A fuse won't:


Operate at its rated current indefinitely.
Blow at currents above its rating instantly.
Protect against very fast current spikes.
Protect against overvoltage conditions if the current isn't abnormal.
Protect semiconductors or ICs against damage - The fuse is slower than the silicon. 
Protect a person against damage - You can still be electrocuted by a fused circuit.
Protect anything if it's on the wrong side of the short.


But it will usually prevent large currents from flowing through it long enough to start a fire.