- When you hold an unbalanced hand, your side's fit is most likely to be in your own long suits, so you need to tell partner what your long suits are. Whereas when you hold a balanced hand, your side's fit will be in partner's long suit.
- Holding an unbalanced hand increases the chances of the deal being a big fit, and so increases the chances of there being a high-level competitive auction.
At the same time, it is more important to show strength when you hold a balanced hand than an unbalanced one. Before going into the reasons for this, we need to look at exactly what it means to "show" the strength of a hand.
The problem with trying to evaluate the strength of a given hand is that everything depends so much on the context - there is a big difference between "offensive strength" and "defensive strength", and both of these are significantly affected by how well the hand fits with partner's. Even so, we have to be able to speak about the "strength" of an individual hand. Without wanting to go into detail about hand evaluation (personally I think that coming up with detailed formulae is neither worthwhile nor particularly interesting), everyone would at least agree that the strength of a hand depends on both the high cards and the distribution. But the value of high cards depends much less on the context than the value of distribution, and so when we talk about "showing strength" this will mostly based on high-card strength.
Now clearly, a 1NT opening bid with a narrow range, such as the Standard American 15-17 HCP, does show strength very precisely. The minimum strength and the maximum strength are both useful pieces of information for responder. However, a bid which has a much wider range, like a Standard American 1C opening (showing 12-21 HCP or so) is still considered to be a good description of strength for hands which are near the lower end of the range. The reason is that partner will play you for a hand of minimum strength unless you do something which shows otherwise. So the fact that you have at least 12 HCP is very useful information for partner - it means that he can bid a game if he holds a similar amount of strength himself - whereas the lack of a maximum is not a problem because he was not going to play you for much more than the minimum strength anyway. This works fine, provided that there is enough space to tell partner when you do in fact have a better-than-minimum hand. (In the case of a 1NT opening bid you obviously don't have enough space to do this, so it is necessary to have a narrow range.)
So, the statement of our new Balanced-Hands-Show-Strength principle is that when we have a balanced hand, we should try to make sure that partner's idea of our minimum strength is as close to the true value as possible.
Why is this so important? Well, as mentioned above, if we have substantially more than the minimum strength we have promised, then partner will not play us for a hand that strong. This would not be a problem if we could make a rebid which showed our extra strength. However, the important thing about balanced hands is:
Balanced hands are not two-bid hands.
As was said in the discussion of One-Bid-Or-Two, the stronger and more distributional a hand is, the more likely it is to be a two-bid hand. Since a balanced hand is, by definition, not at all distributional, it would have to be extremely strong in order to be a genuine two-bid hand.
This hand is a full 19 HCP, but if you open 1C and the opponents reach 3S before you have the chance to rebid, you are faced with a difficult problem. Doubling or bidding 3NT could very easily be wrong, showing that this is not a genuine two-bid hand. But neither is it safe to pass. The problem is precisely that we have not shown strength with the opening bid - partner could easily have enough for game, but he would have no way of knowing.
Fortunately, if we have a very strong balanced hand like this one, high-level interference is quite rare. But still, it's extremely difficult to cope with when it does happen, if we have not already shown strength. With an even stronger hand we would probably have to take a second bid regardless - there comes a point when any type of hand is strong enough to be a two-bid hand - but the vast majority of balanced hands are one-bid hands, and for those we really want to show strength immediately.