I hear signals can be quite valuable in multiplayer, where failing to reinforce (AKA "slipping") on key defensive tiles like El Alemein or the Stalin Line can make or break games. Battles against competent players also tend to be much more grindy and drawn out, making reinforcement checks more important.
In SP, however, losing a few tiles here or there almost never matters, outside of specific achievements like Neither Death Nor Dishonor. You can restart offensive battles to instantly reinforce, and defensive battles are rarely important enough to justify spamming signals in all your infantry. Overall, I almost never see a reason to use signals in SP.