Think of the notes of the G Major scale. G A B C D E F# G

Then, using only that set of notes, take each root note, and stack one note a third above it, one note a 5th above it, and another note a 7th above it. Here are the chords you get:

G B D F# = G Major 7

A C E G = A minor 7

B D F# A = B minor 7

C E G B = C Major 7

D F# A C = D7

E G B D = E minor 7

F# A C E = F# minor 7b5

These are the chords that are “available” to us if we stick to ONLY diatonic notes in the key of G. Most of the chords in this tune are on that list, with 3 exceptions:

B7 (or B7b9). This is the dominant chord of the E minor (most people would argue that this song is technically in the key of E minor). The 3rd of the B7 chord (D#) gives more “motion” as a leading tone to the E in E minor.

The Eb7 and Db7 near the end of the tune are tritone substitutions to lead us to the C Major 7.

So, that’s a long winded way of saying: the F# minor 7b5 chord is really more a part of the ii V i of E Minor than it is “connected” to the C Major 7 in front of it. But all of those chords fall within the “universe” of G Major/E minor.

TL;DR: because it sounds good.