When you connect the large (10 Ga.) wire to the back of the alternator,
You run it up to the starter relay, battery cable side.
There is nothing protecting that wire in the event it gets grounded,
Melted through, rubbed through, ect.
When it grounds, it WILL start a fire!
The Fusible Link is there at the starter relay end of the wire to keep a small issue like a rub through, pinch or getting against the manifold from turning into a full on fire!
As soon as the wire grounds out, the fusible link will burn through sooner, saving your vehicle from a fire.
The general rule for a fusible link is 4 AWG sizes smaller (2 SAE sizes smaller) than the wire you are protecting.
(AWG = American Wire Gage
SAE = Society of Automotive Engineers)
10 Ga. wire, 14 Ga. fusible link.
8 Ga. wire, 12 Ga. fusible link.
Fusible links are about $5 each, available from any parts store, and keep you from burning down or blowing up your battery, melting down your wiring harness, so I recommend them.
Now, some people say you can't have a 'Mega-Amp' alternator with a fusible link...
Which is the usual BS the uninformed pump out.
On average, you alternator will crank out all the power you need without the fusible link ever being an issue.
Since BILLIONS of miles have been hammered on fusible links without people knowing about them, they don't fail very often.
If they were correct (and they aren't) every time you started your engine or ran a winch the starter/winch would draw more current than the fusible link will tolerate, and you would pop a fusible link.
Since they don't know how the system operates, an intact fusible link means your alternator has never produced more than about 30 to 50 amps for very long... EVER!
If it did, the fusible link would blow!
So when most people want to switch to a 'Super Duper' alternator,
I'm usually skeptical...
I'm running a 78 amp that puts out about 70 amps when 'Full Field', (Yes, I have the test equipment) and the fusible links I've installed are still fine several years later,
Even with dual batteries, electric fan, winch, ect.
Your battery (or batteries) and vehicle will NEVER demand more than about 30 amps except when starting,
Which is what the battery is there for, to supply instant current to the high load devices.
The alternator just charges the battery (Batteries) and they take a LOW, SLOW charge or they don't live long!
(the same reason 'Trickle' chargers are good for batteries, while those 200 amp 'Boosters' will kill batteries fairly quickly)
Use the fusible links or fuses to protect your otherwise unprotected wiring to the alternator, and the main feed to your fuse block like the factory used.
Fusible links usually last longer, fuse holders have a tendency to get crud/corrosion in them and stop working, while a fusible link is pretty well sealed (If you install properly with heat shrink) and will live many years without any issues at all.
Fusible links are cheaper, more reliable, available everywhere if you know what to ask for or what you are looking for, and they work GREAT.