If you have a bunch of counters, you don't want a hot counter to require a transaction every time you want to increment a page view. I'm sure there's a solution in high end DBs, but in mysql/postgres UPDATE somecounter SET value = value + 1 typically ends up being very painful for any significant write load. With Redis and Mongo, there are "atomic" increment operations that do a +X on a given field of a list or hash that is very fast.
