Almost 61.5% of all web traffic is non human as of 2013. This non human traffic component, as a percentage of total web traffic, is only increasing.
So non human traffic growth is outstripping the organic traffic growth from real humans browsing the web. Web bots are far easier to build than ever before and as a result spam seems to increase as a function of time.
Without effective automated counter measures, dealing with spam is time consuming, annoying and painfully slow.