Tahoe is aimed at the issues that arise after you buy your 1TB drive which provides local storage quite well. We try to provide data reliability and availability that can survive colocation outages, computer and hard disk crashes, complete local data loss due to natural disasters, etc. Because the encrypted data is distributed over a wide number of servers, it is very difficult to lose data once it is stored in a grid. Also, it allows you to remotely access your data from the web or other authenticated clients.

For our production servers, we can lose up to 66% of all of our servers simultaneously before any data loss will occur. This parameter is tunable such that you can make it even more robust at the expense of more data being stored and pushed around the network.

I can think of several reasons: 1) How often do you remember to plug your USB drive into your laptop to do your backups 2) What if your machine and your USB drive are caught in a fire, burglary etc

I could go on, but whats the point :)