Given the precious scene in which Banquo's death is planned, and the next scene in which the audience could learn with Macbeth that Fleance has escaped, why would Shakespeare choose to dramatize this seemingly insignificant event?

Shakespeare decides to tell the audience that Fleance escapes because he was involved in the witches' prophecy about Banquo, if you paid attention to the witches' prophecy, you must know that Banquo's sons are a crucial part of it. In fact, the main reason why Macbeth wants Banquo and Fleance death is because according to the Witches, Banquo's sons would be kings some day. When the witches tell Macbeth that he was going to be the king, they also tell Banquo: "Thou shall get kings, though thou be none", this means, although you will not be king, your sons will be kings. It was because of this information that Macbeth felt threatened by them, by killing them he could have the crown for himself. The fact that Fleance escapes, tells the audience that the witches' prophecy will become true and that Macbeth will not be able to stop that from hapenning. As a result, the downfall of the tragic hero (Macbeth) begins, because it can be seen that he can't fight against destiny.