It's not the same: date_trunc returns timestamp, to_char returns a string.

rainbowliquor · on June 8, 2017

True but in the use case of OPs example. In the article OP says "So if we wanted to find the count of users that signed up per week:" which "2017-06-05 00:00:00+00" isn't a week, it's a date (with a time stamp as well which isn't pertinent) which happens to be the beginning of a week. Using TO_CHAR() with a format string makes it more legible and more recognizable.