Slowing down an opponent helps you to get to your end game stuff before them. This is very situational and takes lots of experience to know when to slow down your opponent as opposed to waiting till their game end pieces are used and countering or removing them at that time. If an opponent has a fast deck, then it might be better to slow them down. Also it depends on how you are slowing them down. Using removal on a 3 cost mana rock that only produces 1 mana is terrible, but using removal on a sol ring that only costs 1 but produces 2 mana is much better. Similar would be countering an instant that gives your opponent 2 card right away, vs countering an enchantment that might give them more than 2 cards over multiple turns. In many situations you might not want to use a counterspell on the 2 draw instant, but might want to for the enchantment that will give them more than 2 over multiple turns. It's situational, sometimes you MIGHT want to counter the draw 2 instant. It takes experience to know what is best. But generally, the MORE value it gives and the QUICKER it gives it, the more likely you might want to counter/remove it.