Simplesat is unique from traditional form builder tools in that it immediately saves answers after your customer clicks from an email, or directly on the web page. There isn't a "send response" button at the end.
Because of this, requiring fields becomes less important; your customer is free to close the web page at anytime without losing any answers they've already left.
We do understand that there might be situations where you don't want to allow your customer to skip a question without answering the current one. This is not possible at the moment, but under consideration for future development.