Cleaning and boiling just the nipples is much easier than cleaning the whole bottles. I have like 16 silicon nipples (clear color one), and only 2 bottles to use with the 塑料袋.
When you go out, you just need to bring 1 bottle with several 塑料袋, so it's a lot less to carry too.
The silicon nipples (clear color) is more durable and can be boiled many times, but the texture is "harder", so some babies don't like it. The Latex (yellow one) is software but not as durable and some babies are allergic to the material.