Advance warning provides an advantage.

Many idioms that have no obvious source are often referred to, for no good reason, as ‘old proverbs’. ‘Forewarned is forearmed’ has a genuine claim to be called such, as it dates from at least the end of the 16th century, and could be much earlier. The Latin saying ‘praemonitus, praemunitus’ loosely translates as ‘forewarned is forearmed’. There’s no evidence to show that the English proverb is merely a translation of the Latin though. The two sayings could easily have originated independently.

The meaning of the proverb is quite straightforward and literal – so long as it is understood that forearm is here the archaic verb meaning ‘to arm in advance’, rather than the noun forearm, that is, the part of the arm between the elbow and wrist. The saying is so straightforward in fact that it was originally simply ‘forewarned, forearmed’. It is found in that form in Robert Greene’s A Notable Discovery of Coosnage (a.k.a. The Art of Conny-catching), 1592:

“forewarned, forearmed: burnt children dread the fire.”