Quote from: DooMAD on October 29, 2020, 07:32:10 PM

Whichever source your client connects to and starts pulling data from, you know it's going to be the same information everyone else has, because that source would be forked off the network if they attempted to alter the data in their copy of the blockchain.

you don't know this and the node you connect to could actually be on a fake fork. in fact this is a known attack vector.
it is trivial to mine a huge number of blocks on top of genesis block in this day and age with a single ASIC since the difficulty was the minimum. so if you simply trust the first node you connect to and start downloading blocks from it you could waste your time and traffic by downloading a ton of data (the fake blocks could be big too) until at some point your node realizes it is on a fork and has to re-download everything from block 1.
this was mitigated by introduction of headers-first IBD in bitcoin core 0.10