Yep, and even who had the most matches there, Nadal for getting enough clay practice and Djokovic for getting his game more suitable for ATP main draw matches, they had it for their special needs.For those developing faster, the needs may be different. None of the number 1s of the last decade or so spent much time in challenger land. Challenger records:
Others mentioned did not even need that, their general level and confidence grow even in their teens to that magnitude that they judged it as enough to step further into ATP.
Sadly tomorrow's talents have neither the talent (in that proportion) nor the confidence, it seems so far. Maybe Tiafoe is the only exception, though it is still really too early to tell...