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You don't 'need' to convert it, but HVDC transmission is emerging as more efficient and cost-effective than conventional HVAC transmission.

http://en.wikipedia.org/wiki/High-voltage_direct_current

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From what I gather from the Wikipedia page, it's due to the transmission cables being underwater and hence having a much larger capacitance, which is a larger loss than normal DC loss. (Correct me if I'm wrong on that.)

So, while AC is still more beneficial for transmission via cables in air/ground, DC appears to have less overall loss for underwater transmission.

DC is always more efficent to transfer. But it was difficult to efficiently convert to high voltage and back so you only did it when necessary.

The reason underwater links (eg France-UK) were commonly DC is that they linked countries whose grid wasn't synchronised - the same reason long distance grids in the US are DC

By virtue of their tight bundling, all composite multi-core cables have high capacitances. However, onshore AC transmission is typically run on overhead lines, with large line spacings and thus capacitance isn't as much of a problem.

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