It's not necessary but it certainly helps. The opportunity is really to meet admissions staff and students. If you can't travel, then meet alumni, as many as possible.

Visiting campus would allow you the opportunity to get a feel of the place. Some schools that you might think attractive on paper might not turn out to be a fit for you once you visit. I visited a top 10 school back in India, after visiting the campus I just couldn't imagine myself studying there and thus did not enroll.

Additionally, visiting campus will also provide a recognizable face to the admissions committee. I am told that this helps in some schools - might not be true everywhere. Depending upon which stage of application you are in, you can use experiences from your visit in the admission essays/applications to provide the fit factor or you can have a face-to-face interview during your visit.

If you can, do it, especially for the school that's top on your list. I agree, it might be hard to hit all of those stops on a single trip, but if you just pick one and spend a few days there it might not be too bad.

Otherwise, yes, meeting with alumni helps: pick their brains about the experience, campus, student life, and anything else that you'd be interested in. It's much better to get real insight from somebody who's been through the program than read about it from a marketing brochure.