Respect to other chemicals in reduction/oxidation potentials
You cannot define ‘vacuum’ oxidation potentials, or otherwise derive them de novo without having some zero-point. There is, technically, no self-suggesting zero-point around — unlike for example temperature, where $0\,\mathrm{K}$ is equivalent to ‘molecular movement frozen’.

Part of the reason is, that the electron(s) lost or gained during the redox reaction must go or come from somewhere. Therefore, one has chosen the hydrogen cell as a centre of the scale. Why?


Hydrogen is the easiest element with the least electrons.
It’s simple to build.
It actually also comes with a good further distinction: The one between noble and non-noble metals.


You can actually easily recalculate the standard potentials with respect to another half-cell like magnesium: Just add/subtract that cell’s potential with respect to hydrogen to all other potentials. It’s a bit like moving the temperature scale’s zero point from $0\,\mathrm{K}$ to $273\,\mathrm{K} = 0\,\mathrm{°C}$.