Spacing follows the heat demand
Underfloor pipe is not laid at a single spacing across a room. Heat loss is higher near external walls and glazing, so spacing tightens there. The middle of the room does not need the same density.
Lay it evenly everywhere and you get the classic complaint: it is cold by the window. The usual reaction is to raise the flow temperature, which makes the middle of the room uncomfortably warm and wastes energy.
Without balancing at the manifold, circuits do not heat equally
Every circuit has a different pipe length. The short one meets less resistance and takes most of the flow; the long one is starved and its room warms late. That is exactly what the flow-setting valves on the manifold are for.
Balancing is a one-off task but an easy one to skip: the system appears to work and the problem only shows on the first cold days. Recording each circuit's flow at commissioning saves a great deal of searching in later years.
The floor covering is part of the system
Whatever sits on top of the pipe either conducts heat or holds it back. Ceramic and natural stone conduct well. Thick carpet, timber, and laminate laid over an insulating underlay behave like insulation: at the same water temperature the floor delivers less heat.
If the covering is unknown at design stage the calculation is incomplete. Change the covering after the installation is finished and the system may need re-setting — that is not an installation fault, it is a changed input.
The screed cure cannot be rushed
Once the pipe is laid the screed goes over it and has to cure. Fire the system before that time is up and the screed cracks — and in time the crack shows through the floor covering.
Even after curing, the system is not brought straight to full temperature. The first firing starts at a low flow temperature and rises over several days. Recording that schedule settles any later argument about cracking.