If I only have one or a few SKUs that I need to update the inventory count, say I come across a couple of defective items, it is too cumbersome to go through uploading the whole csv file. So I'd go to Product Catalog and search for the product and then click on the arrow to expand to preview my variants. It only shows 10 variants. Most of my products have more than 10 variants; some even have over 60 variants. *If* the variant that I'd like to update is listed within those first 10, great! All I need to do then is click on the inventory count and override it by typing in the new quantity and hit <enter>.
If on the other hand, the SKU I need to update is not in those first 10 previewed variants, then I have to click View All, find the variant, click on it, click Adjust Stock, change the drop down menu to whatever is applicable - Override, Add, Subtract - enter the number and then click OK. So my suggestion is to include all variants in the preview so that they can all be updated with one click, and also another helpful thing would be if I happen to know the exact SKU and type it in the Search bar from the Product Catalog, to have that exact variant just come right up, instead of having to go through the Parent first.