Cascaded Error in Apache Storm
Twitter uses Storm for real-time processing of data.  Problems can happen with real-time data.  Systems might go down.  Data might be inadvertently processed twice.  Network connections can be lost.  A lot can happen in a real-time system.  

They use hadoop to reliably process historical data.  I don't know specifics, but for instance, getting solid information from aggregated logs is probably more reliable than attaching to the stream.

If they simply relied on Storm for everything - Storm would have problems due to the nature of providing real-time information at scale.  If they relied on hadoop for everything, there's a good deal of latency involved.  Combining the two with Summingbird is the next logical step.