The first thing most people talk about when they mention Frederick William I of Prussia is not whether he was an effective leader or not. Instead they talk about his rather eccentric (more like neurotic) personal habits and temperament.
He was born in 1688 and his father was the first king of Prussia, Frederick I, a lavish king who tried to emulate the rich and rather lascivious French monarchs despite the fact that it was not really a good fit with his temperament or character. His son, Frederick William, was a strange sort from the beginning. He was mean, petulant, and strong-willed, once even swallowing a belt buckle instead of spitting it out of his mouth. (A doctor had to remove it later.) He was
[u]ncontrollably violent in temper, vulgar in speech and manner, scornful of education and culture, and so deeply pious that he considered theaters "temples of Satan." ...He made a fetish of cleanliness, washing and grooming himself many times each day.
Clearly he was an odd character who had many areas of his life which were rather out of control and even bizarre. The two things he loved, however, concerned frugality and the military. When he suspected his father's Prime Minister was taking personal advantage of the country's finances, Frederick William was able to prove it to his father and the man was fired. These two passions, economics and the military, are what distinguish him as a successful monarch.
This royal neurotic was the most remarkable reformer of his dynasty....He was the real father of Prussian militarism and Prussian bureaucratic efficiency.
After his father died and was given a lavish funeral, Frederick William became king and he immediately began instituting extremely frugal policies in the royal palace as well as the country. He was able to drastically cut royal expenses as well as encourage farming in formerly un-farmable land (such as wetlands and marshes). He oversaw the construction of grain bins in order to store grain for leaner times as well as flood control structures and wildlife management. He also formulated policy manuals for every member of government, outlining precisely what each person was to do--and the severe penalties to be paid for not doing those duties. While his actions and policies may have seemed rather eccentric and harsh to to the people, they were fiscally sound and created a strong economic structure for his country.
His other passion, the military, also grew strong during Frederick William I's reign.
He gave Prussia an enormous army; one in every nine men in Prussia was a soldier and another 40000 men were foreign mercenaries.
He improved their weapons by adding bayonets to each soldier's rifle and gave them ramrods of iron rather than wood for greater reliability and speed. He hired Prince Leopold of Anhalt-Dessau to help him create a first-class army, including the now-famous marching in formation to make the soldiers even more intimidating to any real or potential enemies.
While his people may not have loved him (and there is plenty of evidence to suggest that they actually lived quite in fear of him), Frederick William I of Prussia was a successful monarch because he gave his country several key strengths: economic stability and military strength. He was an unpleasant man, by all accounts, and he routinely beat nearly everyone around him. He was parsimonious to a fault and cruel to everyone most of the time. Despite that, Prussia prospered and he raised a son known as Frederick the Great.
I would argue that Frederick William I of Prussia was a successful monarch to a large extent. He successfully established his authority in court by overtaking and ruling all aspects of the Prussian administration himself. This occurred at a time when the power of the monarchs was severely constrained by strong local identities and the political clout of the nobility, who often had their own private armies and held much autonomy over the affairs of their estates. By instituting a policy of frugality and selling most of his father’s castles and palaces, Frederick reduced the Prussian budget by two-thirds almost overnight. This allowed him to spend vast amounts of money on expanding and building the Prussian military, a fixation that was to haunt him for life, which made Prussia militarily strong. Institutions that were trans-territorial in nature were established and he sought to centralise control over all his territories through the government body of the Generaldirektorium, that was created in 1722 and made to function in all states. The organisation played the important role of instilling loyalty amongst its subjects to the king. Under his rule, the bureaucratic service was also professionalised. Civil administration was taught at universities and civil servants were put under minute supervision to ensure that they were all carrying out their duties faithfully. The Prussian state grew to become famous for its civil service.