Requests for Evidences (RFE) are not “commonly necessary.” It’s that USCIS officers often have questions relating to the sufficiency of the investor’s case.
USCIS has yet to explain why requests for evidence are "commonly necessary," as the release of USCIS training materials reveals. One can speculate that RFE's are commonly necessary to justify staffing levels, filing fees, and because of ongoing issues that USCIS has in understanding EB-5.
Some of the most common reasons for RFEs issued by USCIS : "an application which is not supported by a business plan and an economic analysis for at least one investment project with sufficient detail to show in verifiable detail how capital investment offerings in the requested industry will create jobs; insufficient evidence or narrative that describes the investment project activities that the regional center will engage in for each requested injured industry category (including the proposed number of investors for the project, the timeline for starting and completing the project, a financial plan and budget including capital required and expenditures for the investment projects)." (source: USCIS)