Necessity of using emitter follower as the first stage
This looks like an input stage of an opamp or comparator. The source impedance will contain also all the external circuits between the Vin or Vip input and the actual voltage source of the circuit which feeds the input signals. There can be for ex. a megaohm resistor as the last part of the  signal feeding circuit. The &quot;superfluous&quot; emitter follower stage can be very useful in that case.
Of course we do not know the design goals of the shown circuit, so we cannot be sure if that &quot;megaohm external impedance&quot; ever occurs. If not, then the emitter follower stages before the differential amp can well be superfluous.