In some formulas, even a pretty small change makes a huge difference in how things behave. For instance, here's the formula I use for making hand-tossed pizza:
100% high protein flour (I use Bay State's Bouncer these days; it's 13% or so)
72% H20 (100 F)
0.1% Instant dry yeast (yes, 1/10th of one percent: one gram per kilo of flour, not a typo. 24 to 168 hours in the fridge makes up for it.)
stir together dry ingredients. Add about half the water, stiring (or using a stand mixer) until the water is absorbed, add the rest in a couple of additions. Takes about 90 seconds using my kitchen-aid and a kilo of flour. Stop mixing, allow to rest a few minutes. Scale into dough balls. put into containers (I use ziplocks, but anything with a good seal is fine.) Put in fridge. wait 24 hours to a week. I do 48 hours to 72 hours, if I can. (I've made excellent pies from a bit that got lost in the fridge for several weeks.)
Dead simple. Except if I use 70% water ( changing the water content from 41.8% of the dough to 41.1%), I get a very, very different dough. It's substantially more elastic, doesn't have the extensibility that's need to make a good pie. If I do that, I still get an excellent tasting crust, but it'll be thicker, a pie with a given amount of dough will be smaller, and I don't like the pie as much. Using a four cup pyrex measuring cup, I can't reliably tell the difference between 700 and 720 ml. Having made the dough a bunch of times, I can do it reasonably well by look and feel, but not as well as cheap scale can. (If I change flours, it's how I do it the first time)
And digital scales are cheap. You can get a perfectly adequate one for $20
Amazon.com: Escali Primo Digital Multifunctional Scale, Chrome: Kitchen & Dining
for example. (What I use, it happens.) And a wonderful top of the line one for $50 or so. If you're working in a bakery, you need something else.
Using a scale has other advantages, too. It's faster. There's less to clean up (particularly nice, if you're using something sticky like honey or molasses). I make a bunch of high-hydration breads that I can get nothing besides a mixing bowl and a wooden spoon or spatula dirty. (well, and the floor. But that's *me*...)
It's much easier to scale a recipe up or down. (That's also an arguement for baker's percentages, as well, but that's a different arguement).
There are probably others, too, but they're escaping me.