Patronage is a system which has its roots deep in antiquity. The most developed accounts we have of this occur in ancient Rome, in which the wealthy and powerful aristocrats acted as "patrons" to their less wealthy and powerful entourage, who were called "clients." Patrons acted as advocates for their clients in legal and political matters, helped them obtain public service jobs, loaned them money, and generally used power and influence in their favor, while in return the clients swelled the patron's entourages, ran errands for them, served as a voting bloc when the patron or patron's friends were running for office, and generally increased their patron's importance by swelling the ranks of their followers. Poets, artists, philosophers, and even entrepreneurs depended on wealthy and powerful patrons for their success well through the eighteenth century in Europe. The problem with patronage is that it meant jobs did not always go to the most qualified person but instead were allocated by means of patronage networks and favor exchanges.
In a highly technological society, patronage, as well as its inherent unfairness, suffers from the problem that technical skills matter. While, for example, as recent scandals show, appointment to the generally honorary position of the Senate (like the British House of Lords) is still done by patronage, for people such as nuclear power plant technicians, bridge engineers, or IT support staff, it's important to hire experts, people who actually know what they are doing, rather than people who contribute money to political campaigns or happen to be friends with the right people.
Starting in the nineteenth century, that has meant a move away from the system of patronage to one of examinations, licensing, and credentials, for example by requiring doctors to complete medical school, pass the Medical Council of Canada Qualifying Examination, and be licensed by the province, rather than just being the friend of someone wealthy or powerful.
The Public Service Modernization Act is part of this evolution of the civil service from a patronage to a professional system, emphasizing training and expertise, and fair and transparent hiring and promotion processes. Part of the change in hiring process of the PSMA was intended to increase diversity in hiring by making it more flexible. Although the older rules-based system had been created to limit opportunities for patronage, fixed rules concerning certain credentials such as schooling or seniority tended to favor white males from affluent backgrounds, and the more flexible approach of the PSMA was partly intended to promote diversity, making the civil service more representative of all Canadians.