Waterfall only works if the programmer knows what the client needs. Usually it goes like:

• Client has a need
• Client describes what they think they need to a salesperson
• Salesperson describes to the product manager what an amazing deal they just made
• Product manager panics and tries to quickly specify the product they think sales just sold
• Developers write the program they think product manager is describing
• The program doesn’t think. It just does whatever buggy mess the programmer just wrote
• The client is disappointed, because the program doesn’t solve their needs