Question

Why is it important for the lipid bilayer to have both polar and non-polar components?

Answer 1

That's a good way to let only certain particles enter the cell.

Answer 2

Think of the lipid bilayer as a wall. You know that walls are grounded to the floor, maybe on dirt.
Well, the wall isn't infinitely tall, but also, a wall, you can let other small animals enter under if they dig a hole.

The wall protects from other humans or intruders from coming inside the castle, but if someone really needed to send a message to this "castle", they can use a bird that flies over, or a cat maybe that goes under. Does that make sense? It is similar.