What type of risk is minimized with the help of preheating a low alloy steel prior to welding?

For low alloy steels, with additions of elements such as Cr, Mo and V, the
CEV formula is not applicable and so must not be used to judge the
susceptibility to hardening. The HAZ of these steels will always tend to be
relatively hard regardless of heat input and preheat and so this is a factor
that cannot be effectively controlled to reduce the risk of H cracking. This is
the reason why some of the low alloy steels have a greater tendency to
show hydrogen cracking than in weldable C and C-Mn steels, which enable
HAZ hardness to be controlled.