In the USA, bolts can come in plenty of sizes, but 2-56, 4-40, and 6-32 are common small bolts. You want metric? You would think 2mm, 3mm, 4mm would be enough

In the USA, bolts can come in plenty of sizes, but 2-56, 4-40, and 6-32 are common small bolts. You want metric? You would think 2mm, 3mm, 4mm would be enough, but no! There are 2.5 mm, 3.5 mm, 4.5 mm etc. Are these 'necessary' even if they can be produced as 'standard'? It's the old saw about Standards -- there are so many to choose from.