When we want to talk about an action that happened in the past, we use the past tense. In the case of "fly", when we refer to an action of flying that already happened, we use "flew". For example, "He flew to Paris last week."
The past tense is an important aspect of language as it allows us to communicate and understand when something occurred in the past. It helps to create a timeline of events and gives a sense of progression in a narrative.