Why do we as Catholics distinguish between mortal and venial sin?

The distinction between mortal and venial sin is a clear biblical teaching:

...If anyone sees his brother sinning, if the sin is not deadly, he should pray to God and He will give him life. This is only for those whose sin is not deadly. There is such a thing as a deadly sin, about which I do not say that you should pray. All wrongdoing is sin, but there is sin that is not deadly....stated in 1 Jn 5:16-17.

The Bible distinguishes between the degrees of seriousness of sin. Bot all sins lead to spiritual death or damnation, and not all sins are equally evil in God's eyes, though, of course, no sin is ever a good thing; otherwise, we would have an absurd scenario whereby a white lie or a momentary pang of jealousy or lust would be the moral equivalent in God's eyes of torture, rape, or murder.

There are also numerous examples of different rewards and merits in Scripture, which suggest diferent levels of sin, as opposed to a stark "sinner versus saint" perspective.

(Mt 16:27, Rom 2:5-13, Cor 3:8-9, 1 Pt 1:17, Rv 22:12)