That's to secretly impose throttle or slowdowns without you even knowing.

Just like in US where old legacy AT&T unlimited subscribers holding on to their dear life, gets throttled to slow speeds occasionally without getting told by the telco since they claim it's based on fair usage.
This means the telco will get to slow you down as and when they feel like it's "fair" to them to sell more lines.

Sicne many are complaining of slwodowns, just to tell you all it's better to have high quota than to be UNLIMITED with the "fair usage" condition.

With high quota, you still get to argue that as long as you have not finish it you can expect best effort speed where attainable.

Unlimited fair usage means the telco can slow you down when they feel like it according to how much they want to oversell their lines. They will argue with you that it's unfair to have it all for yourself since you're expecting unlimited.

According to your contract they can't totally cut off your line, but only throttle you after exceeding the quota. But we know their main intention is to REALLY cut your line when you hit it so you will buy more quota.So how?

Make your line slower than dialup to frustrate users to the point that their internet is near to useless so they will move and buy more quota lor.

They can't cheat you so they squeeze you!

At least if you didn't exceed their quota, you can screw them entirely for not providing enough coverage or capacity in your area, etc.

If they find out it's unlimited they always use the fair usage as an excuse to say they have no choice but to limit you for others in the area to enjoy as well.

You see it's not that they actually care whether their customers are suffering with congestion they are just interested in the money.

So as you can see their own meaning of "fair usage" also refers to their own interests of providing "fair" profit margins instead of subscriber experiences. I can tell you none of the telcos will think for their subscribers except for their own pocket.