Most of them are run on a vps server so that they can be used round the clock. its easy to use. there are a lot of EA's available out here , but most of them are not good to use. EA do trade for you even when your PC is powered off. These kind of things are having their own limitation as the main decision should be taken by the traders only from their own brain. If you have an EA that works with a functionating strategy it wont be function if you have not the right broker.