Being able to buy or sell a specific quantity can be a nice feature for those who want it, htough it can probably also be annoying for those who would prefer to be the one choosing the quantity.
For example suppose I am thinking I might like to buy a special deal on alpaca socks but would need no more and no less than 100 BTC to do that. Meanwhile I also am tempted by a collection of phonograph records someone wants no more and no less than 2000 USD for.
I would thus like to buy either 100 BTC or 2000 USD or both, but any less of either is useless (I might as well just use my GBP for whatever I normally use it for and forget about those socks and recordings.)
While it is nice for me to place an order for no more nor less than 100 BTC, I can see if being potentially irritating for other people who might wish they could sell me some smaller quantity.
They would be able to since orders are not filled one to one, but on a fifo bases matching the best orders that completely fill the order, so with enough volume both orders would be filled.