Speed and reliability of connection to the network. The droplets are, as per the way things normally happen on SAFE, the same as other nodes with except for being hard-coded. They are the same with regards to storage, and there would be no loss of privacy from using them.
The data, in the case of SAFEproxy, is webpages retrieved by the user’s browser.
Think of the droplets by (a very rough) analogy with bittorrent indexing sites: If you know a download is out there somewhere that you want, you could just fire up your bittorrent client and, if you knew someone already well-connected, bootstrap to a cloud of similar clients and search their kademlia table for the download. But hardly anyone does that. Instead they get a torrent file from an indexing site. A Maidsafe droplet network would serve a similar purpose, of getting you well-connected to the much-larger cloud of peers. Once you’re there you don’t need the droplets, although even then they might help with speed. I’m not sure how authentication could be done in order to only allow paying clients to bootstrap from the droplet network.