If you're visiting Hoover Dam you must walk across the bridge too. The parking area is right before you get to the dam, you can take a ramp all the way to the top or you can take the quicker stairs path. It's a decent hike up.
The bridge spans the Colorado River below and provides enormous views of the Dam and Lake Mead behind it, great for photo-ops, total time to park, walk up the stairs and get to the middle of the bridge is around 10 minutes.
You can enter Arizona by walking across the entire bridge, you can take a photo in front of the Welcome to Nevada and also the Welcome to Arizona signs, crossing the bridge will also change your time zones, Mountain on the Arizona side and Pacific on the Nevada side.