If you're referring to septic tank treatment products, they aren't always necessary for a healthy septic system. A well-maintained system with regular pumping and proper waste disposal should function adequately without them.
However, if you're having problems with your septic system, such as slow drains, clogs, or backups, then a septic tank treatment product might be a temporary solution until you have your septic tank pumping inspected and pumped by a professional.