Whereas before I might need four read slaves to ensure that at peak load I'm handling all my transactions within X ms, now I can guarantee it on one server.
More importantly, in our industry where we're vastly more write-constrained than read-constrained and we're faced with e.g. MySQL not being able to easily spread writes over multiple servers simply, the appeal of something like MongoDB or Cassandra with built-in sharding and rebalancing to spread out both reads and writes sounds very appealing.
And again, I can move from a giant complicated, expensive, heat-producing multi-disk raid10 to a pair of SSDs in RAID1 (or better) and easily meet my iops requirements.
Without being able to upgrade to SSDs I think we would have been looking into other systems like Cassandra a lot sooner, but right now we can pretty easily throw some money at the problem and it goes away.