After partner deals and passes and RHO passes, why would it make sense to open light (in 3rd Seat)? Of course, all normal hands with 12/13+ points will be opened. That is not the issue here.
There are many hands with which you would pass in any other position, but should open in 3rd seat. For example, after 2 passes, you should open 1 with:
Why? Because it makes it harder on the opponents! It is much more difficult to find the right contract when you are starting out against a 1 opening as opposed to having a free run. Maybe when you open 1, your LHO has, say:
Perhaps he is vulnerable and fears a 2 overcall. Meanwhile, his partner has:
It might be tough for the opponents to enter the bidding--yet they have an easy 3 contract their way. On many occasions, your light 3rd-seat opener will cause your opponents to miss a laydown game. They might get into the auction, but might not be able to appreciate their fit/values.
Simply put, bidding is much harder when the opponents have struck the first blow. "Defensive bidding" (responding to takeout doubles and overcalls) is much tougher than when your side gets to open the bidding.
Lastly, it is "safe" to open light in 3rd seat. Your partner is a passed hand. He can't (shouldn't) bury you. He won't have enough to get your side too high. The advent of Drury (a popular convention) is also a big aid in putting on the brakes after a light 3rd-seat opener.
When (and with what) should we open light in 3rd seat?
Not 5 points (unless you are preempting). Probably, you should have at least 8 or 9 HCP for a light opening.
As usual, vulnerability and suit quality plays a big role. Here are some examples:
After 2 passes, both sides are vulnerable and you hold:
Open 1. This will make it much harder on your opponents. You will get a spade lead (good) if partner ends up on lead. Partner (a passed hand) won't get you too high.
Pass. You have spread out (defensive) values and a poor suit (which you don't want partner to lead).
Open 1. Yes, we assume 5-card majors. But, in 3rd seat, try this space-consuming, lead-directing action. Partner isn't going to get you too high and meantime you really mess with the opponents' auction.
After 2 passes, you are white against red (favorable vulnerability), holding:
Open 1. This makes things much tougher on your opponents than a pass. So what if you end up -50 or -100? Likely it is their hand. Go for it!
Open 2! Yes--with only 5 hearts. Partner can't do too much damage and you are white on red. Make it tough on the bad guys. As Marty Bergen says, "Third seat favorable is not bridge."
Open 1. Get in their way, get a heart lead and take advantage of the favorable vulnerability.
Summary: When in doubt, open light in 3rd seat. It makes life much tougher on your opponents. Be wary when your partner opens in 3rd seat--tread lightly as responder.