Telling a story situated in the past by using the present tense is a stylistic instrument to increase the immediacy for the reader. The temporal separation is removed and the reader feels more deeply immersed. This is also an option in English, by the way:

"You won't believe what happened to me the other day. I'm walking down the street, minding my own business, when suddenly someone taps me on the shoulder."

The first sentence sets the temporal stage, indicating that what follows took place in the past. Then the narrator switches to the present tense to draw the listener into the story.