The essence of the system, in place by 1601, was that each parish would tax its wealthy inhabitants and give money to the needy poor.
England was uniquely successful in establishing this system across the country, by the 1640s everywhere from London to Northumberland.
By the 1690s, redistributive tax was capable of feeding around 5% of the population, a major achievement for an early modern state.
This undoubtedly helped England become (probably) the first country to permanently escape famine (the last one was in 1623).
It may have helped prevent popular rebellion, and by ensuring people were relieved in their home parishes, it reduced the spread of disease.
Most interestingly, the historian Peter Solar suggested that the safety net provided by the poor law allowed ordinary people to take risks.
So, if they wanted to sell the farm and become - say - a cotton weaver or iron-worker, they knew they had a social security safety net.
For similar reasons, it may have helped allow internal migration (thought this is hotly debated), to industrial and urban areas...
But more generally, it helped provide valuable security for people at a time of unstable marketisation and industrial growth. So, yay!