Hollow Needles are most commonly found in medical environments alongside syringes to infuse and take out liquids from the body. Often these types of tools are formed from stainless steel because they are required to be strong and as hygienic as possible.
Stainless steel fits the above factors as it is a strong metal that is resistant to corrosion and easy to store and keep clean. If they are going to be used to both extract and inject liquids from and into the body it is vital that they are clean. Preventing infection is the best form of defence and a corrosion resistant material helps to play a major part in insuring this.