The higher your total hashrate the faster you mine and get paid.
So if you stack 10 of these you have 50gh/s which will get you more back faster than just one.
The same goes for any type of miner. More, bigger, faster the better.
Why do people buy multiple of the same product? Does it become ineffective quickly?
The reverse is also true. If you purchased badly and spent more on the device than you'll ever earn back effectively making a loss, then buying 10 of them means you make 10x the loss.