In the Star Wars prequels, Padmé Amidala wears an insane amount of regal and ornate outfits. In The Phantom Menace alone, she wears at a minimum nine different outfits. For comparison, (if memory serves) Princess Leia wears eight outfits in the entire original trilogy. I have read the canon book Queen's Shadow, which explains the in-universe reason for her having so many clothes, but what was the out of universe reason for making her wear all those clothes?
This is discussed in the Star Wars Episode I: DVD Costumes Featurette. In short, Padmé's outfits create a feeling of grandeur, sophistication and elegance (as befits someone of her noble station) and are intended to 'wow' the audience.
Trisha Biggar (Costume Designer): When I first started in the film, we were going to have three costumes for the queen. And, as time went on George decided that every time we saw the queen, that she was going to have a different costume.
George Lucas (Writer/Director): Somebody of that stature would automatically be changing her costumes to fit the occasion. She travels, she goes to the senate, she has all these official functions and non-official functions, and each one of those demands a different type of costume.
In the other films I've been involved with, it's always been a very simple wardrobe. This became a very complex, very designed, very fashionable wardrobe.
Rick McCallum (Producer): I think people will be amazed by the costumes. The images are so haunting and so beautiful and I think it's something that they've never seen or witnessed before in any previous Star Wars film