Many of the manufacturers want constant circulation of the HFO to assure it maintains a constant viscosity and to prevent drop out of heavies and contaminents.
HFO can do some strange things at times, especially if is residual fuel, it gells, drops out waxes, gets pockets of aromatics, and sometimes generates hard deposits in low flow areas and fittings. So keeping it hot and moving minimizes those problems.
Hope that helps