Hello,
I hope I won't look too silly posting this

I'm going through my Sybex book, and I've even bought lots of hardware including serial connections, but I'm wondering why we use mostly serial connections to connect the hardware together (instead of over Ethernet/FastEthernet connections, etc.) I also saw another post with someone trying to use T1 connections instead.
For home lab purposes, does the speed of the serial matter? async/sync (a/s), normal, high speed (hssi)? How about with regards to CCNA, CCNP, CCIEs?
Thanks in advance!