You can keep alpacas for a number of reasons. You can use them to keep the grass down in a paddock if you want a relatively low maintenance grazing animal.
You can use them to guard chickens or sheep from predators, an alpaca will adopt other species and protect them from foxes or similar, both of our labradors have been evicted from the paddock at times by a fired up alpaca!
You can breed them for the wool, although you will need quite a number to make this viable.
Finally, they make great pets, they are impossibly cute and once you have a couple it is very hard to resist getting more.
Go on , get the alpaca habit! (Apologies to the Abbey National!)