Why did Johan van Oldenbarnevelt become landsadvocaat of Holland in 1586?

Johan van Oldenbarnevelt became landsadvocaat (chief minister) of Holland in 1586 primarily to serve as the trustee of the political inheritance of William the Silent following his assassination.1

His appointment was also a strategic move to safeguard the individuality and special position of the province of Holland. At that time, Robert Dudley, the earl of Leicester, was serving as governor-general and attempting to impose a centralized government on the various provinces, which threatened Holland's autonomy. By assuming this leadership role, Oldenbarnevelt was able to mobilize and coordinate the country's resources, effectively supporting the military efforts led by Maurice of Nassau while maintaining Holland's influence within the United Provinces.1