To be more specific, I need to know the link between Kaiser Wilhelm I. and Bonn. Why was the monument placed there? Was it because of his glory or where other factors taking place?
It is entirely normal for cities in empires to erect monuments to emperors. The exact practices vary between countries, but there seem to have been at least 425 to Wilhelm I erected between his death in 1888 and 1918. It was unusual in Prussia to build monuments to living royals, even before the creation of the empire, so there were few before his death.
A more meaningful question would be "Who decided to put up this monument to him in Bonn?" That was likely just a group of patriotic local citizens who could raise the money, and obtain the agreement of the city's authorities and permission from the then-current Kaiser, Wilhelm II.