12.26Why A7 sounds good after Cmaj7

Cmaj7:

C E G B

A7:

A C♯ E G

Several notes stay or move minimally:

E stays

G stays

C → C♯

B → A or C♯ depending on voicing

That single:

C → C♯

strongly announces:

D is coming.

Then:

C♯ → D

into Dm.

Excellent voice leading.