Whenever you train your players, 10% of it goes to a random skill and and if the random skill is at a high sub-level, it will pop up. This is called a "cross-training pop." This is why you center popped in OD from IS training.
Now for the usefulness of Good outside defense on a big man. Big men with good secondaries will sell for more on the transfer list than one with poor secondaries. Also players with good secondaries are usually more salary efficient and perform better in games than their counterparts with poor secondaries.
Even though this game is dominated by the look inside tactic, there are still teams that run outside or balanced tactics. In my opinion, outside defense is a bit overpowered and a having a team full of great outside defenders makes it harder for the opposing team to find good shots. It disrupts their offense flow. Also having a PF or C with good outside defense will make it easier to defend outside oriented big men.
My last point is that good secondaries on any player provide you with a better level of flexibility when it comes to tactics. If you see a weakness in the other team, you can adapt to it and create mismatches.