Underscarves are available for everyone, though not every muslimah wants to wear on. It is not compulsory to wear a underscarve if you can manage without it that is absolutely fine. The main purpose of underscarves is to ensure that if your hair is constantly coming out then you can actually take action to keep it in place. However, silk, satin and occasionally some chiffon hijabs can be very tricky with wisps of hair coming out and you having to struggle to put it back in place. They are also very handy when you want to actually pin your hair into place as the pin will then attach the underscarve with the hijab.
underscarves are great for providing full coverage under see through hijabs, but please note that apart from full underscarves and ninja underscarves most will not provide coverage for the neck or chest area.
Some muslimah like to wear the underscarf similar to earrings, they see it as an accessory to pump up their look, so you can do this by creating different looks and because underscaves are value for money they are always seen as a added benefit.