Importance of length (i.e. quantity)
Honours look nice, but each can only win once
When a suit is going to be chosen as trumps, remember even the lowly 2 will beat the Ace of any other suit when given the chance. And when you do have long suits, you also tend to have shortages elsewhere, which is just what you need to have the chance to beat these enemy Aces with your trumps.
In any case, your Aces are generally going to win whether they are trumps or not, as long as you have most of the trumps.The more you play, the more you'll realise how important it is to have length.
Having said that, it's generally not so great to propose a 5-card suit which has nothing higher than the 9. That's the kind of suit you want to discard while you are playing. On the other hand, if your partner proposes that suit, then every one of those 5 low-value cards in your long 5-card suit turns into pure gold.
No Trumps
A long suit is helpful with "No Trumps" too. If you can establish a long suit, or better still 2 long suits, then your No Trump contract is going to be much easier to make. Your opponents will end up sadly and painfully discarding their high cards on the long tail of your long suit, with no means of gaining the lead to play their big cards.
So, when opening the bidding, bid your longer suits first. There are one or two exceptions to this when you are responding to a partner's previous bid, because sometimes it's better to bid a Major before a minor, since getting to Game in a Major is easier.
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