Because when you take the jargon out we are talking about optimizing damage and accuracy.
Edit: In fact I will take this 1 step further... I am a DM that worries about Optimization but I don’t worry about feats. So I am your target audience. Tell me in plain English why I should worry about feats in general or the -5 to hit + 10 feats in particular.
OK. Since Froggie is disinclined to help out, this is a quick explanation of what is going on:
They are talking about a specific optimised Fighter build, designed to max out damage.
The Fighter is a Battlemaster (BM) with the Precision Attack maneuver. This allows you to roll a superiority die and add it to an attack roll, increasing your chance to land a hit. The fighter also has the Archery style which grants a +2 to hit with ranged weapons.
The character has two feats, one from variant Human race and one from level 4.
Crossbow Expert (CBE) The important bit here is that when using a hand crossbow, the character gets an extra attack as a bonus action. (Also useful is the ability to carry on using the crossbow in melee.)
Sharpshooter (SS) The most important part of this feat for the build is the ability to take a -5 to hit to get a +10 to damage on an attack.
So; basically this build is designed to get the lots of attacks (from CBE), all with a +10 damage (from SS), while using the Archery fighting style and the Precision Attack maneuver to mitigate the -5 to hit from SS.
Why level 4?
This is not only the first point at which this build gets all of its functions, it is also the point at which the difference is greatest. Level 3 and the build would be missing one of the necessary feats. Level 5 and the build would no longer be getting twice as many attacks as the comparison example. It would only get 3 compared to 2.
Basically Frogreaver ran the numbers comparing a level 4 fighter with this build with a level 4 Sword-and-Shield fighter who had just picked up +2 Strength at level 4. They worked out that in order to bring the featless fighter up to the level of the optimised handcrossbow fighter, you would have to give the featless fighter a longsword that did an extra 2d6 damage. (For example a Flametongue weapon).