How much yarn to purchase depends on the project itself. Usually you will find this piece of information on the pattern you are following. There will be an instruction on which yarn brand to purchase/skeins to purchase.
If you are making a small coin pouch/small toy/ coaster, yes surely that 50g of yarn will just be enough. However for bigger projects the chart below is really a helpful guide to know as an estimation for each bigger projects. This is really important if you want the whole piece to look the same. Especially for lace/cable techniques, you would want the stitches to appears the same. 
At the end of the day, One do need to remember we are creating a piece of textile fabric using the techniques of knitting/crochet. This piece of "Cloth" needs to have enough materials/bulk to take shape.
Just like baking, one needs to have enough flour/sugar/fats in it to make the final product.
The advise is: When in doubt, always buy an extra one.
📌This is especially important if you are yarn shopping overseas/ online.
Sometimes, after you return to your home country, you may not be able to locate the same brand you had bought during your holidays. Also, Online sales moved quickly and sometimes the item may be out of stock and you may not get back the same dye-lot again.
I will explain more of these scenarios below on what happens when one don't purchase enough.
After running a yarn shop for many years, one of the common thing I see Knitters/Crocheters do is running around the whole of Singapore to get back that SAME yarn they had in the first place, as they had RUN out of it.
Below are the Most common reasons I've encountered when one runs out of yarn:
1) the wrong estimation of yarn needed for the project.
2) when the yarn was purchased, the crafter intend it for one project, but then may have changed their mind/pattern and started on another project, which may require more yarn.
3) NO STOCK available on the "web shop"/ other yarn shops that they had first purchased
4) Had stored the project away for ~10+ over years, and finally decided to do it, then realized that not enough/ yarn had already discolored.
And many had the expectation of buying back the SAME YARN to complete the Half-done projects.
Many a time, Expectations vs Reality is very very different... ...
📌Honestly, I have to tell you this brutal truth, If you didn't buy enough in the first place, chances will be you will not be able to find back the SAME yarn to complete the half done projects.
All of these above scenarios can really be avoided if in the initial stage one plans to purchased enough. Unless one don't mind to change the colour/fiber type/thickness then this is ok.
Below will answer:
📌Why you may not be able to find back that SAME YARN you have purchased before?
Reasons being:
1) The wide variations in yarn itself.
Besides colour, Yarn in itself is categorizes into various yarn brands, thickness and fiber types.
This is like 4 different kind of variations already.
For colour; different yarn brands will have dyed their yarn color hues different from another brand. Which means their Hues may differ slightly, NO one company will have the exact dye colour. There will always be a slight difference. In a way, this is actually the beauty in it, cos of one's perception, hence, the hues differs right.
Below is a great video for understanding in dye, just by using paint.
Moving on, Even within the same company; the dye lot can be different as well. Which means, batch by batch (even with the same yarn fiber and thickness) is dyed differently in the same company. Hence this can cause a slight variation in the product, even though the colour code is the same.
You can see below picture for this differences. The upper one looks much whiter. 
2) The Stock in various yarn shop is different.
As yarn shop owners, we also will stock different brands and items.As the stock differ from Yarn Shop to Yarn Shop, chances will be the fiber types and thickness is different as well.Hence if you have purchased previously from Shop A with Brand X, chances will be Shop A still has it. No point to run to Shop B, cos Shop B may not stock this brand at all.
In any case, do always check in with the shop before running all the way down to prevent a wasted trip.
3) Existence
Like many other business, Yarn Brands can fold up and no longer in business.For Knitters/Crocheters who came away their work for a long time, chances will be they may not be able to complete the project using the SAME yarn.
Sometimes, the yarn you purchase could also be a limited special edition, and like many other limited edition merchandise it was a "limited time" purchase only, hence you may also not find back the SAME yarn again.
So can you see it is really impossible to buy back the EXACT Same yarn if you didn't purchase enough in the first place.
જ⁀➴The take home message is, always buy enough and plan ahead to avoid running into these situation. 
I will write up more on "What to do If I run out of yarn?" so you can have a read if you run into this situation.
