On the following hand you open 1♥, partner raises to 2♥ and you have to decide whether or not to try for game.
You know partner has between 6-9 points. What more would you really like to know about partner's hand? Would you like to know whether partner was minimum or maximum for the 2♥ bid? 6 points or 9 points? That would be some help, I guess. What if you could find out whether partner had the ♦A or ♦K! Now that would be useful! If partner has a fitting honour for your diamonds it will really help establishing that suit.
In the two specific sequences 1♥ - 2♥ and 1♠ - 2♠ you can bid a new suit to invite partner to game if he has an honour in that new suit. Bidding a new suit like this is called a 'trial bid'.
Partner says yes. Game makes.
Partner says no. 3♥ is high enough
Let's look it from responder's point of view. I'm going to rotate the cards so you're still South.
Partner has invited to game. Accept or decline?
It's close. 2♥ showed 6-9 points and you have 7. The ♥A looks good because that's partner's suit. How good is the ♦K? You have no idea!
That 3♥ bid from partner gave you no clue as to how to evaluate your side suit honours.
Aha! I know my ♦K is worth something.
Not so keen.