the reason that no power is supplied to the encoder chip until a button is pressed is to save current, but also, if you look at the circuit, the 4 words get encoded by bringing pins 9 and 10 hi or lo,
sw 1 pin 9 = Vcc pin 10 = 0 V
sw 2 pin 9 = 0.6 V pin 10 = 0.6
sw 3 pin 9 = 0 V pin 10 = Vcc
sw 4 pin 9 = Vcc pin 10 = Vcc
so if the chip has a full time ground, it will always be in sw 4 mode, and transmitting that word, which will keep whatever relay is attached to the decoder chip on all the time, every time the power sw is turned on, this is not good,
so by having the chip ground going through the switches, the word attached to sw 4 will only be enabled when the button is pressed.
now there is another reason for keeping the transmitter off when no momentary sw is pressed, this is a cmos encoder, it runs from 2.5 volts to about 18 volts, even with the ground lifted, voltage is mysteriously appearing across pins 8 and 16 (pwr supply pins) when this happens, the chip puts out some noise on the transmit pin which is triggering the decoder which turns on some of the output pins , there are some leakage currents somewhere that are creeping in to the encoder chip, have not figured out how, so if you hook the transmitter ground to the encoder ground, power is only available to the transmitter when you press a button. this keeps current draw low if the pwr sw is left on by accident (only the LED indicator is on which will last for 2500/.-5 = 5000 hours) .
here is the current path which shows that sw 4 does not reach pins 9 and 10 but only grounds the encoder pin 8 thru the diode.