I've had chickens, for 55years. Never fed grit. Never had but one sour crop. Saved that one easy. Bagged grit is all money for the rock company. 1 cent of grit, sales for 5.99$ .All profit.
Don't confuse anecdotal experience with empirical evidence, or suggest that just because what didn't happen to you won't happen to others these are fallacies... Not everyone's setup is identical, thus there is no single fit all answer or solution, what works wonderful for some very well would be a total disaster for others...
Fact is not all terrain is the same, it varies greatly by region, sure in some areas there is suitable grit to be found, but that doesn't apply everywhere or to everyone... Also the amount of range is important as well, if your chickens are confined to a run area or even small yard they will deplete that area of suitable grit in time... In my area for as long as I can remember when they do new housing developments they strip all the top soil, grade with clay then put down maybe two inches of screened top soil and seed or they lay the sod over the clay resulting in just a few inches of soil then clay... This this doesn't exactly leave ripe land for the chickens to scratch and obtain grit or even bugs from, especially when most neighborhoods have a large percentage of people spraying this or that on their lawns...
Hand full of sand from outside any where. Turn um out in the run. They will kick up their own grit.
Sand is not the appropriate size for adult chicken grit, nor are the 'round' pebbles found in construction sand optimal for grit... If there is no grit on the surface ground in your location it's not going to magically appear because the chickens are kicking around...
As for it's worth or profit margin, I personally don't care, it's worth the $5.99 to me, as it's almost a certainty that I make that amount and than some back in better feed conversion due to them having proper grit, plus $5.99 is a drop in the bucket in the overall expenses well worth the piece of mind that they have suitable grit at all times even if nothing else...
I get it other people have a fend for themselves philosophy, are too cheap to spend the money or don't think it's worth the money, and to them I will have to beg to differ on those ideals...