In Endgame we are shown that the Time Travelers leave from a transmitting / receiving unit. But this seems optional.
When Tony and Steve are in 2012, they leave from there to get to 1970 - no transmitter or receiver in this jump. However, 2014-Thanos needs 2014-Nebula to rig the device to ensure his arrival in 2023.
My question is - what role does the transmitter / receiver play in the time travel, when is it needed and when not?