The logic of using ladder line (window line technically here) is that impedance of halfwave dipole is around 70 ohm, fullwave dipole is about 5000 ohm, and if you use feedline that has impedance close to geometric mean of these two, then if full-sized dipole is used worst case SWR is about the same for both of these cases, about 15 tops, and you can use fundamental + harmonics, and even some others. In contrast if you tried to feed full-wave dipole with coax SWR would be closer to 100 which is not workable at all. This arrangement strictly requires tuner with balanced output
Impedances of ladder line make it possible to make a match in some cases just by varying length of this 400-ohm transmission line. If length of feedline is halfwave or multiple, it'll present the same impedance to transmitter as antenna has. If it's quarterwave, it'll step it down (450^2/5000 = 40.5 ohm = 1.23 SWR; 300-500 ohm is practical) so both odd and even harmonics can be used even without tuner if you can vary feeder length. In fact i've seen someone did just that by putting lengths of ladder line spooled on something and bunch of relays to get just that effect
As there's less plastic in ladder line than in coax, everything else equal, losses are lower, but everything else is not equal. In particular, if you have matched antenna fed by coax with low SWR, losses can be lower there than in ladderline antenna with constant SWR of 10 or more. Both are useful in some situations and both have merits. I don't have tuner, so if I had space for 40m dipole, I'd make an OCFD for that band (makes 40m, 20m, and 15m work) and separate 10m band antenna (because it's wide and multiband antennas aren't likely to cover all of it). Separately, WARC bands (12m, 17m, 30m, maybe also 60m if space is available) can be covered by trap dipoles (these tend to have narrow bandwidth, but these bands are narrow too so it's fine)
Window/ladder line is kinda hard/expensive to get (300 and 450 ohm are commercial; real impedance is somewhat lower), so you can make your own. Because it's just wire with spacers, feeder and dipole can be made from the same stretch of wire, without soldering