Because he is able to impose control over enough land and tribes Mieszko I is considered the founder of the modern Polish state. Because he converts to Christianity — and specifically Latin rite Christianity — Mieszko I places his state within the sphere of Western European culture and religion.
It is thought that one of the reasons why Mieszko I converts from paganism to Christianity is that western Christian rulers, especially those of the Holy Roman Empire, would no longer have a right to invade and conquer Polish territory on the pretext of converting pagans to Christianity. In a sense, conversion to Christianity means that Mieszko I is joining a club with special perks and protections.
Another effect of the conversion to Christianity is that the traditional class of pagan priests is completely disempowered, leaving a vacuum which Mieszko I is able to fill with his own people and eliminating a rival for the people's loyalty. The incoming Christian clergy will be completely independent of traditional tribal and family networks. While they won't be blindly loyal to Mieszko I, he can use them against other tribal leaders.