The Case for Support is the single most important document in a project grant application because it is the document used by the grants committee to decide whether or not to award a grant. The task of the Case for Support is to make a convincing argument (case) that your project deserves funding (support).
Most of the committee have very little time to read the Case for Support and are not experts on your research topic, but their collective assessments carry more weight than those of the one or two more knowledgeable committee members who read it carefully.
Referees, who are experts on your research topic, also read the Case for Support and send written reports to the committee. Bad referees’ reports will kill a grant, so the Case for Support must contain the technical detail that referees expect. However, a Case for Support that impresses the referees and is impenetrable to the committee is very risky because the referees do not make the decision.
The committee, which is dominated by members who have have very little time to read the Case for Support and no expertise in the immediate subject area, discusses the Case for Support and makes a collective decision. Consequently the Case for Support must excite readers who are unfamiliar with its technical language and have very little time. But it must also impress the referees who read it carefully and expect to see technical detail.