Why is winning a hand in hearts or spades worth more than winning in clubs or diamonds? I can understand extra points for winning in no trump, because that requires different tactics. However, playing, say, 3D is functionally identical to playing 3H, but winning a hand in 3H nets more points than winning a hand in 3D. What gives?
It's because hearts is higher on the Clubs->Diamonds->Hearts->Spades->NT ladder, meaning you can bid over 3D with 3H but must bid 3H over with 4D. If all of the suits had the same point value...
Let's take a hypothetical situation. NS pair bids 3H. EW pair bidded 3C before, and now have the dillema between passing and going up to 4C. If they lose, it'll be by one or two tricks - no big loss, maybe even smaller than what NS would win with 3H. If they, by some strange occurance, win - they get a nice point bonus(over 400 points overall, if I remember correctly)
It's to introduce further levels of strategy in the game. By making the minor suits worth less per trick, it opens up an additional layer of consideration. Is the hand such that 3 no-trump is more likely to succeed than five of a minor suit? Is a sacrifice going to be worth it due to needing to bid one level higher? Alternately, how worth it is it to sacrifice against a minor suit contract? Figuring out these minor details are the difference between the good and the great.