Remember Steam Machine?
Steam Machine is a Water card, but it grows with Fire. As do Fire Spirit and Fireeater. Lava Golem is a Fire card as well. Forest Spirit is the only exception to the needs-Fire rule.
A spell that lets any creature grow is a stronger card than a single creature type that grows. It's like the difference between Anubis and Quintessence.
Incidentally, Otyugh is perhaps not that far off from growing on its own but for obvious reasons is not really the same thing.