HIV/AIDS is a serious infectious disease, and one method of transmission is the use of a contaminated needle. Someone that gets HIV that way will not know it, and may not get tested, putting them in a position to spread the disease through continuing to share needles and sexual activity. Needle exchange, which offers clean needles to prevent disease transmission, can stop the spread of HIV and other bloodborne pathogens, so it saves lives and doesn't hurt anyone.
Needle exchanges can help reduce the risk of diseases such has HIV and hepatitis. The needles are not illegal, and it's really no different than providing them to diabetics. Addicts are going to use with or without needle exchanges, but this would reduce tax payers footing medical bills for addicts that catch and spread disease, and may help save lives.