Smart Previews surfaced for one primary reason: RAW file size.
Smart Previews are essentially small RAW files, re-sizing the RAW file down to a copy with 2048px max width. Keeping full sized RAW files on a server is impractical due to their large size, but there are other local advantages, too.
Each time you make an edit in LR, it has to "consult" the original RAW file and generate a new JPEG preview. If you keep your RAW files on a HDD, and your library on a SSD, it's spooling up that HDD each time you make an adjustment. This use of energy is increased if your RAW files are external, on a NAS, or in the cloud. This small size allows quicker updates as it "consults" the smaller RAW file, which, due to their smaller size, Adobe is hoping is more local.
This means that you have a good gain from using these files locally, even if you're always connected to your RAW files. More efficient preview updates means better performance. The only time this is not true, is if you keep your Lr library and RAW files in the same location. Lr will then consult the original RAW and you are wasting space with the Smart Previews.
This opened the gates for Adobe to take your RAW files into the cloud, put RAW files on tablets, etc. These are the benefits that Adobe is pushing since these are the benefits that bring in more $$ by selling subscription based Creative Cloud.