God sends Death to summon Everyman because he's lost his way in life. He's become so preoccupied with earthly possessions, with material wealth and riches, that he's been diverted from the path of righteousness. The arrival of Death is intended to be a wake-up call, a chance for Everyman to change his ways and get his life back on the right path again.
In the days before the Reformation, Christians believed that it was possible to achieve salvation through one's good deeds. It is essential, therefore, for Everyman to accompany the allegorical figure of Good Deeds on his Christian journey if he's to be saved. Because when Everyman dies and finally stands before God on the Day of Judgment, all he'll have with him will be his good deeds.