It's not just for neatness; either. Alignment shows where differences are between similar lines. And not necessarily consecutive lines. For instance, if we rapidly flip the editing window back and forth between two buffers showing similar code, we can see the differences as the moving parts in a two-frame animation.

Even in proportional fonts, the digits 0 to 9 get the same width. Why? So that tables of figures will look reasonable.

We don't want 1111.11 looking narrower than the 100.00 in the preceding row.

I want everything to be crisply aligned between lines like:

   xr = x * cos(theta) - y * sin(theta);
   yr = x * sin(theta) + y * cos(theta);