Do you know why they help us breathe?
Becasue they create oxygen and help sustain an atmosphere.
If we don't have this atmosphere we get swept up into space and eradicated by solar radiation.
We need plants to breathe, but we need a lot more than plants to remain alive.

Ever notcied that Mars was never famed for bein' the planet that has a lot of plants?

S'becasue there's none. We can't put plants there becasue there's no atmosphere. We can't make an atmosphere because there's no ecosystem. We can't make an ecosystem becasue plants die due to lack of an atmosphere.
Vicious circle, get it?

Therefore, we need to create artificial atmospheres.

Unless you have a way of growin' plants on Mars w/o all of that junk...millionaire overnight if you do, btw.