Here's a simple example why it's important to count HCP before making your lead.

Your hand:


K1098
10983
AQ4
AJ


LHO  PardRHO  You
PassPass1NT*Pass
3NTAll Pass


*15-17


The opponents have shown about 25-27 points here. You are looking at 14, so what does that leave partner? 1 HCP at most. Therefore it is futile to lead a spade, even though it's your best suit, because partner cannot have the ace or queen, and he probably doesn't have the jack either. It is dollars to doughnuts that the lead of the spade 10 will blow a trick; think of all the possible spade holdings declarer and dummy can have:


AQ3

J42


Declarer wins the lead in hand with the jack and then leads toward the queen.


A52

Q6


Declarer wins the lead in hand with the queen.


Q5

A2


Declarer puts up the spade queen at trick one.

The best opening lead on this hand, therefore, would be the heart 10. A windfall of heart tricks for the defense is hardly expected; but the lead of the 10 from a three-card sequence is extremely unlikely to cost a trick. Leading from any of the other three suits would be horrible. It is often just as important to avoid blowing tricks on opening lead as it is to try establishing them.