In an ideal scenario, the user is so good at prompting that they can reach a great outcome on their first try. Realistically, that's unlikely to be the case. This is why wayfinders are so critical as patterns: they help users get to great outcomes with less friction.
Follow ups catch users on step two, after they have given their initial prompt to the AI but before the AI has started to build its response.
The intent is not to hide a long questionnaire behind an open input, a dark pattern that takes a seemingly simple input and makes it unnecessarily complicated. Instead, follow ups are short clarifications to build trust by showing the user that the AI is on the right track, or let the user intervene proactively.
Successful follow ups act as sample prompts, borrowing context from the initial request, and giving the user the sense that the AI is moving with them. Consider combining follow ups with a plan of action to provide even more upfront transparency and control to the user.