What is a reason for using an engagement stream for multiple emails, versus the "wait" feature in a flow? So, if I wanted to send email #2 three weeks after email #1, why wouldn't I use the flow "wait" option versus an engagement stream with both emails and cadence set for three weeks?
Because it is easier to drop in new content and turn old content off in an Engagement. I wrote about this here:
essentially smaller programs and variable cadences are better with regular Programs and wait steps.
Lindsey, with engagement programs you get additional metrics for your email performance - specifically, engagement score. You can also do multiple streams within an engagement program and compare performance as a whole. Also, it's easier, IMHO, to take people out of engagement stream and put them back in where they left of (i.e. if they do something and you want to put them through a different flow but then add back into the original drip).