Since no one can keep the law, what is its purpose?

10 as it is written:

There is no one righteous, not even one.
11
There is no one who understands;
there is no one who seeks God.
12
All have turned away;
all alike have become worthless.
There is no one who does what is good,
not even one. —Romans 3:10-12

We are incapable of perfection

12 Therefore, just as sin entered the world through one man, and death through sin, in this way death spread to all people, because all sinned. —Romans 5:12

We are incapable of escaping our condition

20 For no one will be justified in his sight by the works of the law, because the knowledge of sin comes through the law. —Romans 3:20

We need Help

The law shows us we are not perfect, we cannot escape our condition, and that we need help.