E-W have an 8-card Spade Fit but 3/5 pairs played in 3NT, 1/5 in 3♥ , and the fifth played in 4♥ .

With only 23 High Card Points this hand lacks values for Game unless you play in an 8-card or better trump fit when the shape adds the Strength for game.

Once East rebids 2♥ then West knows that East has at least 5♠ , whether they are playing 4-card or 5-card Majors, and hence knows they have an 8-card fit.

With 11 Points (and LTC=8) West's next bid should be an invitational 3♠ , which East accepts with the extra value from the short Diamond and Club suits.

Not every well-bid game makes though, and this is one such case with the defence bound to take two Trump tricks and the two Minor Suit Aces.