IMO the only purpose of external nestboxes is for people in very small AND VERY LOW (reach-in or tractor) coops, where an internal nestbox would take away too much walking-around space from the chickens. They can be quite valuable in that situation.
For larger coops, or for coops tall enough to mount the nestboxes at least 14" above the bedding, IMHO there is absolutely no sensible reason whatsoever to use external boxes. (Although in some cases a person might want an external *hatch* to collect eggs from *internal* nestboxes without entering the coop).
In large part I think many people build them either because they have seen them and think they look unique or 'chicken-y', or because they have just not thought through the issues involved.
JMHO, good luck, have fun,
Pat