The electrical current that a wire will carry, governs the thickness of the wire. Also, the higher the voltage used, the less the current needed to transfer the same amount of power.
Low current and thinner wire also mean a lower cost. Its why the power companies have very high voltage power lines. Running a thick 12v cable 12m is a much higher cost due to the amount of extra copper in the cable.
If for example, you are using 10A at 12V (or 120 watts of power), you would need about 3.57mm dia cable for that 12 metres. (see table)
To run 240V that 12m and carry the same 120 watts of power, you only need about a 1mm dia cable to carry the 0.5A
Other considerations that come to mind:
240V MAINS can be a lot more dangerous and should be restricted to licensed people if doing anything other than running a ready made extension lead.
You should also not run 240v AC cable next to low voltage DC (on long runs) or next to data cables (ie. USB cables) as they could pick up signals/noise from the adjacent 240v AC cables.
Note the attached table is from a
Victron battery cable guide.