The Romans did not invent it. They were the first to construct aqueducts the way we know them:

* the water of the Tiber, the river that flows through Rome, was too dirty to be used as drinking water. The Romans invented the best drains in the world. They had underground drains to take away dirty water and sewage. They were flushed through with water from the baths so that they didn't get too smelly. But ... this waste water flowed into the Tiber!
* Roman engineers objected against the use of siphons.
* a conduit high in the air would safeguard the water against theft and pollution.

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