The rule is you cannot start a word with any rising diphthong.
The reason I heard was at one time u was usually written v, the Latin way. So without an h you'd see vevo, veso, vérfano which could lead to lots of confusion.
It seems to me as well that in older Spanish ie at the beginning of a word was written ye: yerro, yelo, yedra
Putting an h standardized the treatment of rising diphthongs, even if it created irregular verbs like oler, but extended h to helar....
Starting words with rising diphthongs is also seen an anomoly in Spanish, and sounds weird to natives especially when it's unaccented. Over the certuries many of the words that had diphthongs lost them. One of them that comes to mind now if enero which previously was *yenero-ienero-hienero. In recent times the hui/huo/hue has been strengthened by g to get rid of the diphthong sound so hueso might sound like güeso, even loaned words like whiskey can be pronounced güisqui.
You might ask this in the etymology forum too