Let me quickly paint the picture of two Salesforce training events:
The attendees who arrive have never used Salesforce before. The instructor asks them to login but several haven't received their "welcome" emails. The instructor spends the first 30 minutes trying to get people set up and logged into the system.
Nobody is the least bit familiar with how Salesforce works so the instructor has to keep repeating instructions to make sure that everyone is in the right spot.
Everyone feels overwhelmed by the amount of new information and the information retention level is low.
All attendees arrive at the session already having logged into Salesforce so getting everyone up and going takes just a few minutes.