The fb and bf knots are used because it is very easy to create complex designs like that.
This type of pattern is called 1212 pattern. In it the strings alternate between the background colour and a colour of the design. Here the background is a gradient and the colour of the design is yellow. By alternating the strings and only using fb and bf knots, any knot can have either the background colour or the colour of the design. This way you can 'draw' any design you want.
@gazjewtinx usually they're only on the sides to make the final bracelet straighter, but with this one its just the pattern it is(i couldnt figure out a way to make this pattern without all bf/fb knots lol), nothing really changes as far as im aware! (also thank youu <3)