for start up. They work like this:
When not started, vref=0 and vbias=Vd(M5)=vdd
M6 is cut off, M4 should be a PMOS and act as a resistor, so Vg(M5)=VDD
Then M5 is On and pull down vbias to make current mirror and amp work.
Vref begains to rise and make M6 on and M5 off.
Hi
yes these transistors for start up
some times the circuit does not need but also use it tto make sure of circuit working after fabrication
there are other methods used for the start Up for example there is method making use of the PWD control