[MoDOH]

An openraie from CO doesn´t say much about the raisers hand as he could pretty much raise any cards here trying to steal the blinds.
A reraise though usually means a pretty big hand and therefore earns more respect, as it should.
So with a strong hand as KJs OTB against a CO openraise a reraise should be in order against aggressive (LAG:s?) players like these. You will more often than not get it HU and often you will win the pot right there.
When it´s HU you are IP and you have the initiative, a powerful combination in NL-Hold´em. And will more often than not win the pot with a bet on the flop.

So the upside of rerasing is that.
1. you might win the hand right there.
2. You get the pot HU and gain the initiative so you can win the pot postflop UI.
3. You might fold out a better hand, KQ and AJ for example...

The upsides of just calling is that the blinds are still relatively small and with a hand like KJs he is perhaps hoping to flop something nice so he can take down a big pot. By calling you induce the blinds to tag along too creating a multway flop.
since these guys are so confident in their postflop abilities he might figure that with the best position and a decent hand he might as well see a flop and take it from there...