The proverb is used to express the idea that when someone is desperate and has no other options, they may be willing to break the law to obtain what they need.
It could be used to justify the behaviour of a man who steals food to keep his wife and family from starving. As Cervantes wrote in Don Quixote: Necessity urges desperate measures.
The proverb originates from the Latin phrase “Necessitas non habet legem”, which was first recorded in the Decretum Gratiani, an important collection of canon law compiled in 1140.