back in the day, people had to estimate distance by measuring the width of the target in mils and then doing some math, aka "milling".
it was very difficult to do with round and oval mils in the original "mil dot" scopes, so companies started putting fine hashes in various places on the reticle. different strategies included .2 mil hashes and dots everywhere (see several in post 14 on prev page) or putting .1 hashes on the edges (see the G3 in post 14). the latter was a poor strategy because the edge of the glass usually has a lot more distortion than the center and it makes it more prone to error, PLUS, you have to be a lot more steady/still to measure the size of a target than you do to actually shoot it. i can wobble around quite a bit and still hit targets, but i have to be still to measure them. so getting a good NPA on the edge of the glass, and then having to move 10+ mils away sometimes means rebuilding the position a bit. it's time consuming. so the closer to the center of the glass you can measure something, the better. that's why the reticle walkalong posted improved on the G3 by moving the fine hashes in to around mil 4 instead of 8.
the genius of the gaps in teh reticle i posted is that you can measure everything on the line at every mil. for example, between the major mil hashes you have one small hash at .5 and a .1 gap from .2 to .3 and .7 to .8. so, from the major hash it's
.2 to the start of the gap
.3 to the end of gap
each dash is .4
.5 to the small hash
.6 from gap to gap inclusive
.7 to the start of the second gap
.8 to the end of the second gap
.9 is two dashes and a gap
these days, pretty much everyone has an LRF, but there are still times to use milling. LRFs aren't perfect, and sometimes you can have rolling hills etc and devious match directors will put targets on the hill tops so they appear next to each other but are in fact maybe 200 yards apart. or you might just have trees or bushes in front and behind the targets. a good LRF will typically return two ranges and you'll have to pick which one is correct. if you know the size, you can quickly mil the target and know which reading from the LRF to use.
but the more common use is when spotting, to give corrections to a teammate. being able to quickly measure the distance between where the bullet was supposed to go and where you saw it actually land, using the reticle, then conveying that to your teammate for quick 2nd round hits is a key skill.
edit: 2 decades ago i started a thread for practicing... all the pics have disappeared now though
https://www.thehighroad.org/index.php?threads/ranging-practice.539037/ (back then, THR had a white background so i made the answer text white so it would be invisible until you highlight it, but now not so much)