You can build real useful systems on them. For example, Gmail used to be implemented on top of GFS as two append-only files per user: the zlog and the index. The zlog held (zlib-compressed) operations such as "add this message" and "delete this message". The index was an append-only btree as described here. It was a true index in that it didn't include the actual message contents. Those were simply pointers into the log. Thus, it was much smaller than the zlog and its efficiency was less of a concern. Also, periodically both files were "compacted" by writing a fresh log (that built the current state in the most efficient way) and matching index. This was primarily for privacy (otherwise your deleted messages would never actually go away) but also improved efficiency.

Gmail's now built on top of Spanner so uses a log-structured merge tree. Still append-only files but a bit different. Files aren't per user but per some arbitrary key range boundary; no more btree; multiple layers with the top ones getting compacted more frequently; etc.