If the partner has a 5+card major suit, the partnership can play a contract in that 5+card major suit. The opener will have at least 2-cards in that suit (and most likely 3 or 4 cards). The partnership has a 7 or 8-card fit (most likely an 8-card filt), so the partnership can play a contract in that major suit. The Transfer process creates the opportunity for the opener to mention the desired major suit first, so that the opener will be the declarer.

Partner's Requirements
• Suit Length: The only requirement is a 5+card suit in a major (hearts or spades). If the number of hearts and spades are equal, bid the higher ranking spades. Ignore a long suit in a minor.
• Point Count: There are NO point requirements. The partner does not need points in the 5+card suit, nor does the partner require any points in hand. A "bust hand" with zero points and 5+cards in a major is all that is required for a transfer response to a  3 NT  opening bid.