You're able to gauge the temperature in your relationship
Sometimes couples will still have sex even when there are issues going on. Others even use it to numb themselves of a toxic situation.
If usually you would spend more time together after sex, but then of late you've just been rushing through the process, it will tell you that there's still some problems that need to be dealt with.
If you're not okay, you might still want to have sex but when that's done, it will be hard to force yourselves to go on like nothing is wrong and the tension and anger will still be there.
It's a way of healing
There are many people who have gone through traumatic events in their past. For them it takes a lot of effort to think of sex as a positive thing and they need more love, patience and care.
In this situation, it helps to spend more time together after sex. It helps them feel safe, which can encourage healing.
It makes you happier as a couple
Couples who shower together, stay together. It's true because when you do your post-sex after-care together, you bond more.
If you think about it, there is a difference between how a one-night stand is handled and when you're in a committed, loving relationship.
In the latter, you will want to laugh together, take a warm shower or maybe even go for another round. The point is that there will be no rush to avoid each other after sex because what you have is real love.