I'm very new to the Phantom, but you ask WHEN might you ever need manual?
If it's even remotely like the Hubsan, then the amount you can tilt into the wind would be greater in manual mode, and therefore you MIGHT find if the quad was in high wind, and struggling to get back to you, then you might be able to fight against the wind a little better in manual? However, as I said, I could well be wrong about what "manual" mode is in a Phantom. My comment is made based SOLELY on flying a Hubsan in "Expert" mode (I fly it that way more than I fly it in normal mode) so I might be totally wrong about how the manual mode works in a Phantom, but if it lets you get a greater angle on the quad, it would in theory give you greater power against winds. However, you'd also lose altitude, so you'd probably want to be practiced at it BEFORE you find the wind is stealing your quad, and you are thinking about trying out manual to get back.