Whereas SIPs express the epistemic attitude of their (logical) subject concerning the degree of certainty of the embedded event, predicates like want or offer denote the role of their subjects as either having some degree of responsibility on the embedded event (e.g., promise/offer to go; force somebody to go), or being in a greater or lesser favorable state towards its accomplishment (e.g., need/want to go).
they express distinctions within the space of deontic modality.