When you're running a webapp it's the primary cross-platform storage facility provided. It sounds like he's doing some buffering and bulk writes to optimize the use of indexeddb rather than just passing through each query to try to mask some of the slowness of the end storage.

It is what available for persistent storage. IndexedDB run queries very slow but throughput is good enough so rather big chunks of data stored in single cell to offset latency. And IndexedDB is just one storage backend. Author says he experimented with webkitFileSystem but didn't matched IndexedDBs througput. Point of this project is storage medium abstracted so in future it will be replaced with what's better.