I saw that on crunchyroll, i was wondering if i should watch it. Is it any good? xD
Anyway im no expert but there are a lot of advantages when its 4-5-1. Firstly the most important thing to consider is the midfield battle, with more people in central midfield (3 compared to 2 in your normal 442) it becomes much easier to control possession of the ball and through that control the tempo of the match. Having 3 midfielders also give you the luxury of having on of them focus exclusively on defending. The deep runs from midfield by one of the three midfielders is also an advantage. Its much harder to mark or defend against a player running late into the box, someone who is an expert at that for example is Frank Lampard. He times his runs into the box perfectly and from there picks up loose balls, deflections etc and from that, scoring.
Also the positioning of a lone foward can be advantageous. If the striker drops deeper into midfield it creates space in the position he was occupying so one of the wingers can cut inside and attack that area. Barcelona do this amazingly, Ibrahimovic would drop off and messi & co will drive into the empty space. A striker dropping off also poses a dilemma for the defenders. Should the defender follow him and mark him but leave the space behind him open? Or should they stay in position and face the danger of the striker getting possession in front of them?
There is more like having that midfield player focusing on defence helps the fullbacks go foward much more easily, and it also is a good strategy to use against counter-attacks. These runs are helpful in getting the wingers to cut in and overlapping with the fullback. In fact, just watch Barcelona everything they do is what im basically saying. They have one foward but are one of the highest scoring teams in world football. In fact they broke countless records in goalscoring two seasons ago. So if its played correctly one fowards can be better than playing with two.
Though that formation can also be played negatively. Like having all 9 players simply focus on defending and leave that lone foward up front to do nothing but bother the defence. This is mostly used when a team believes they will lose. It had a bad reputation because of that over here in england. Anyway in club football in Europe most top teams have started to play with one lone foward. It getting more harder to see teams that play with two fowards in the latter stages of the champions league. The formation helps the team stay more solid defensively (1 extra midfielder focussed exclusively on defence) while at the same time helping them keep possession easier and if played well it doesnt take anything away from their attacking. So its becoming more common. In fact in the world cup final both teams played with one foward. Im not sure if i explained that well

Anyway here's a
link by a football writer discussing it.