In Goblet of Fire, there is a Muggle named Roberts who is in charge of giving out campsites. Why did the organizers need him? They could just take any place and enlarge it by magic, without risking Muggles finding out about magic.
It seems pretty clear that the Roberts' campsite was simply located in a convenient location near to where the Quiddich World Cup was taking place.
Presumably the sizeable influx of guests from around the world would make it necessary to use all sorts of muggle services for the supply of food and water (neither of which can be magically created) and especially lodgings. It's notable that the Diggory family is assigned to the next field over, one owned by a Mr Payne.
He picked up the dusty kettle and peered inside it. ‘We’ll need water …’ ‘There’s a tap marked on this map the Muggle gave us,’ said Ron, who had followed Harry inside the tent, and seemed completely unimpressed by its extraordinary inner proportions. ‘It’s on the other side of the field.
Out of universe, obviously the Roberts' family serve as a proxy to show how badly the wizarding world (both goodies and baddies) treat muggles. The Ministry are enchanting his memory a dozen times a day and the Death-Eaters are so despicable that they choose to harm his family for the sheer fun of it.
I would guess it was just for ease of use - there was no reason to co-ordinate and employ a magical logistic system when a perfectly good muggle one existed.
Possibly it was an ethical issue. If the field was hired out by the owner and the Wizards wanted to use it then it is only fair that they follow his rules ... although I am not so sure this is a big thing in the wizarding world!