This last point is an additional benefit of the semi-bluff in stud games but especially in hold 'em. When you do hit the card that makes your hand, your opponent will often misread it because of your bet on the previous round (except in the cases where you were straightforwardly betting on the come with a flush or a straight draw). Thus, you may win a larger pot than you would have otherwise expected.
Now let's put the examples together. It's fifth street. The situation appears reversed and the other person has three hearts exposed on the table. You're holding the aces from the first example, and you've been lucky to catch a third ace. You're trying to push him out, and he seems to be trying to keep you in. The two of you may be locked in a little betting war if you don't fold. Of course he may not have the flush. Look around the table. If you see nine cards with hearts then he will never have a flush. If you don't see any hearts, his chances are good, maybe better than your chances of catching the fourth ace or a pair to make a full house. He's looking tentative. This might be a ruse. The smart move may be to fold. Or maybe not. Is your opponent the kind of person who frequently bluffs? Maybe those two down cards are spades. Now multiply this scenario by every person at the table.
Both the semi-bluff and betting a marginal hand rather than risking giving a worse hand a free card are cases of the general precept that it is usually better to be betting than calling. By betting as a semi-bluff you have a chance of winning the pot right there, something you are usually hoping to do, and you have shown greater strength than you really have. If you catch scary-looking cards after you have been called, you are still likely to win pots you wouldn't otherwise have won. When you bet now, your opponent is quite likely to fold. On the other hand, when you don't improve and are caught in a semi-bluff, that can be of value as an advertisement for the future.
Players gather information about their opponents' hands by analyzing betting patterns, noting exposed cards, and by reading player tells. These are unconscious movements or body positions that indicate what a person is thinking and therefore playing. A tell can be an ear scratch, a lean forward, or a heavy sigh. It can be the way someone fiddles with the chips or sips a drink. Some people nonverbally scream, "I'm holding aces!" Others telegraph the message "I've got trash! " Poker pros see this and use it.
A final advantage of the semi-bluff, is that you can sometimes use it to get a free card. Let's say an opponent in hold 'em bets on the flop, and you raise with a four-flush. If that player calls your raise, it is likely he will check to you on fourth street. If you haven't made the flush, you have the option of checking behind him for a free card.