Here's a tough auction for the responder (playing old fashioned standard). With no interference, 1--2--3. That's invitational, but say you have 

 KQ5  
 97643  
 K2  
 765
 ?

You're in the middle of your range with decent shape and trumps. Game would be good opposite, say 

 AJ1093  
 AJ  
 AJ43  
 82
 . You would be able to ruff two diamonds and might make an overtrick if the heart finesse is onside. If you switch partner's suits around so that partner has: 

 AJ1093  
 AJ  
 82  
 AJ43, 

Now the game is terrible with a heart loser, one or two dimond losers, and two likely club losers (or even three if you're not careful). Having your honors fill in your partner's missing values goes a long way to making this hand better. Describing these invitational hands is 

Once we have a major suit fit, we don't go looking for another fit. As a result, if a major has been bid and raised, none of the bids of new suits need to be natural. We can't play in diamonds if the auction starts (unimpeded) 1--2, then 3 (or, for that matter 3 or 3) can not be non-forcing. We don't want to play there. If those bids are forcing, we can use them to describe our hand to partner without fear of ending up there.