A PC is ideal as long as there is only one. Once you start adding machines, it is time to move to a rack. Otherwise you end up with a giant ball of wires. Racks have rails like drawers, so the units are easy to service. Thats not the case with a heap of ATX towers. Also, your "collection" remains self contained and doesn't grow. Past some scale, you start to also make use of other rack mountable accessories like UPSes, PDUs, patch panels and switches. In the past, you had to move pretty early because older consumer machines like Pentium4 could only do so much, so you needed many of them even for a basic setup like a LAMP server or render/compile cluster.