Multisite installation question

I currently have four blogs running on my primary host. One is in the root, and three are on subdomains. I have two questions about multisite installation.

1. What would be the benefit to me of switching these over to a single multisite installation, rather than leaving them as individual installations? (I’m looking for specifics, as I don’t fully understand how the multisite works yet.)

2. Do all four blogs have to be removed and reinstalled? Or can the multisite option be activated on the primary blog and then only the other three imported? (If I don’t have to reinstall the primary blog, that would make a huge difference in how much work/time this takes.)