A manual contains all the possible answers to all the possible questions in a single document. To get an answer to your question, you just have to read, re-read, assemble, interpret and then you have your answer. It could take you 5 or 10 minutes depending on the quality of the manual and the complexity of the question.

With SO, you bypass all that and simply get an answer. You don't gain any understanding of the bigger picture, but then again, maybe you don't need to. Maybe you're diving into say esp32 to build your smart doorbell. Your don't want to become an esp32 expert, you just want your question answered.

Horses for courses