2.2Why G major needs F♯

Let's build a major scale from G using the same formula.

Start:

G

Need a whole step:

G → A

another whole:

A → B

half:

B → C

whole:

C → D

whole:

D → E

whole:

E → ?

E → F is only a half step.

We need a whole step.

So:

E → F♯

Then the final half step:

F♯ → G

Therefore:

G major = G A B C D E F♯ G

or:

соль ля си до ре ми фа-диез соль

This is why F♯ exists in G major.

It isn't an arbitrary notation rule.

It is required by the major-scale interval pattern.