The rationale behind the "Major Contributor Report" (MCR) requirement is to add an element of transparency to large infusions of money in the month immediately before an election IN WHICH THE COMMITTEE OR CANDIDATE is active.
Basically, providing voters with information about the source of funding for any "October Surprise" -
it provides no useful information to voters (or anyone else) but adds a not insignificant compliance burden for those committees NOT active in the odd-year election.