Parquet is columnar storage, which is much faster for querying. And typically for protobuf you deserialize each row, which has a performance cost - you need to deserialize the whole message, and can't get just the field you want.
So, of you want to query a giant collection of protobufs, you end up reading and deserializing every record. For parquet, you get much closer to only reading what you need.
Parquet ~= Dremel, for those who are up on their Google stack.
Dremel was pretty revolutionary when it came out in 2006 - you could run ad-hoc analyses in seconds that previously would've taken a couple days of coding & execution time. Parquet is awesome for the same reasons.
