Why migrate command checks for child DocType customizatrion before sync?

While exploring bench migrate command, I came across this the following condition

for doc_type in doctypes:
	# only sync the parent doctype and child doctype if there isn't any other child table json file
	if doc_type == doctype or not os.path.exists(os.path.join(folder, scrub(doc_type) + ".json")):

According to this condition, frappe only sync customization of parent and child DOCTYPE, if there is no other customization for child DOCTYPE exists.

Can someone explain the purpose of this condition here?