I cannot stress enough the importance of our presence there. If the followers of Jesus Christ won't stand up for the unborn and for their mothers, who will? Our Lord commands us to help the weak and the helpless. Abortion has taken root in this country because too many of us have been indifferent and it will maintain its grip on our nation unless we finally decide to stand up and be counted on the side of life.
A Priests for Life brochure explains the importance of taking the pro-life issue to the streets :
Those who support abortion will sometimes say that they defend our right to "have different beliefs" and "express our views publicly." Aside from the fact that the actions of most pro-abortion groups contradict that assertion, the point is that our presence on public streets and sidewalks is about more than expressing beliefs and viewpoints. It's about saving lives. It's about stopping bloodshed. It's about intervening to save a victim who is about to be killed. When violence is being committed, victims do not simply need people who will "express their belief" that the violence is wrong. They need someone who will come to save them. While much needs to be said (and is said here) about our First Amendment rights, pro-life activity has an even firmer foundation than the First Amendment. The foundation is the fundamental obligation that human beings have to care for each other and, insofar as possible, intervene to save one another. (Source)