I have been told that Ethernet magnetic transformers are used for base-t Ethernet when transmission is sent over a lengthy cable. What is the purpose of the transformer? (is it for signal filtering or boosting?)

Additionally, I have a circuit that has been used for an 8 wire (1000base-t) Ethernet configuration. Would the same circuit work for a 4 wire (100base-t) configuration if I connected only pins 1, 2, 3 & 6? if so would the performance be the same as the 8 wire configuration?

I'm sorry if I may not be coherent in my question as I am not too familiar with Ethernet hardware. Thank you for reviewing my question.

P.S. I am unsure about how data is transmitted over Ethernet. Is it that all differential pairs (DA: pins 1 & 2, DB: pins 3 & 6, DC: pins 4 & 5, DD: pins 8 & 7) are busses where data can be transmitted both ways (unlike the UART where RX has to be connected to TX)? and in case I am just using 2 pairs, would I be connecting only DA and DB? Is there also an issue with connecting a 4 wire device to a network that uses 8 wires?