You watch demos to "judge" your decision making and whether your "read" of the opponent's was on point.

As CT, watch out for your positioning. Your positioning on how to defend a bombsite has a huge impact on how fast you can rotate and on the other hand how well you can defend a bombsite.

The best example would be the B bombsite on inferno, when you have the "role" of the rotator. The best way to defend the site, is when both defender are staying on the site. When playing from truck or spools, you can easily get smoked or flashed off, so your "impact" on the defense is quite small. The only way to help your team mate in this situation is with grenades or doing a risky push through a smoke. However when it comes to rotating, you have a much longer travel distance to A, when you stay on site together with your mate. So you have to judge the attack pattern of your opponent's and whether you should "priotize" rotating or defending.

Also have a look at your grenade management. You should not throw away all your grenades early and in the best case, save them until the very end of the round (if you consider buy rounds). On the other hand against eco rounds, it's a better idea to use some grenades earlier to stop rushes, since that's the most powerfull option for an ecoing team.

The same goes for T site. In the best case scenario, you safe all your grenades until your go for the "execute" on a site. However you have to "judge" how you fair in aim battles against offensive CTs. If you find your team down by 1 or 2 players early in the round quite often, it's better to use your grenades early for map control to build up "confidence".

A good example is dust2 A short, when CTs are agressively peeking lower tunnels. There will be games, when you or your team can easily walk through lower tunnel and click heads or avoid damage by agressive CTs. However in some cases, the CTs will win this fight more often. In this case if you are a teammate in upper mid, it's a ood idea to flash short, so your teammate in tunnel can easily clear this part of the map (support role).

Overall when watching a replay, you judge your decision making and question it and you analyze how impactful you were in this game.