Why Method of Moments (the math behind NEC-based modeling programs like EZNEC) breaks each wire into many short segments and assumes the current on each segment is constant. The program then builds a matrix of self and mutual impedances between all segments and solves it simultaneously for the current in each one, and those currents give the radiated field, pattern, and feed point impedance. Accuracy depends on using enough segments, typically at least 10 per half wavelength, with segment length short compared to a wavelength.
Watch out The choices about a single sine-wave generator or voltage source describe the old textbook assumption of a sinusoidal current distribution on a dipole, which is an approximation MoM does not need; the voltage-per-segment wording inverts what the model actually solves for, which is current.
Moments = many segments, each carrying one current value. Think 'chop the wire, solve the currents.'