What is the most important function of the rainstorm at the beginning of the play? to create a believable space for liza and higgins to meet to provide a way for higgins and pickering to be able to meet to foreshadow the importance of the hill's later roles to comically portray higgins as cop?

I believe the answer is : to create a believable space for Eliza and Higgins to meet.

In the play, all of the main characters are met/brought together because of the rainstorm. Freddy is looking for a taxi for his mother, liza is trying to sell flowers among people who seek taxi,the colonel buy liza's flowers, and Higgin is taking notes on Liza's accent.