Ordinal value is different than numerical value. Sure, all cards have numbers on them, but the number on the card doesn't always tell you everything.

Suppose, for a moment, that you have six clubs: the 3, the 4, the 8, the 9, the 10, and the Queen. Which card is the lowest? Well, actually, both the 3 and the 4 are, because neither can be underplayed. Want to tweak the opposition? Play the 4! The lowest club is always on everyone's mind, and you'll keep them off balance if it's still out there.

Going back to that hand, the 8, 9, and 10 all have equal ordinal values too -- they're all 1 bigger than the 7 (they beat it) and 1 smaller than the Jack (they lose to it).

Adjacent cards in your hand always have the same ordinal value, because they can be played interchangably. So what's the value of knowing the ordinal value of cards? You confuse your opposition. People tend to play in patterns, usually playing the lowest card possible and trying to lose most tricks, other times playing the highest card possible to win a trick. But, since the ordinal value of a card depends on what's in your hand, said value is inherently unknowable to your opponents. That 8 and 10 may be interchangeable to you but they're not to your opponent, and your opponent will wonder why you tried to win a trick with an 8 -- and how you got lucky enough to pull it off!

Let's try another example. Suppose you have the 3, 4, and 5 of clubs. You know that two players are void clubs, because the last club trick showed that. You'd like to flush the Queen of Spades but you don't have the spades to do it with. Say that you have a lead in points and so the player with the Queen would like to lay it on you. Now, you could play the 3, but then your gambit would be obvious and the Queen would never come out. But, if you play the 5, the Queen-holder may take the relatively good gamble that the other player has the 3 or the 4 and can underplay you, and discard that Queen. Good result for you when your opponent takes the trick with, probably, their highest club (since they're stuck taking the trick, they might as well dump a future winner and take fewer discarded hearts down the line)! The Queen is out and you're safe. Better, the two people who were void in clubs, the two people who didn't know that your opponent has no choice but to take the trick, are now wondering what's going on! Could your opponent be shooting the moon? They'll play safe and take points themselves to prevent this -- just what you want, and all because you realized that the 3 and the 5 had the same ordinal value.