Well, the posters before me have already answered the question, I just want to make it a bit more clear.
Imagine you are a photo professional and you make your living by processing digital images in Photoshop. A 2.7 CPU will allow you to process a few images more per work day (given that your workflow is flawless), because filter application etc. will be faster. Thus, you will be able to complete your orders quicker and potentially earn more money. Of course, if you only get few orders in the first place, the faster CPU won't save you anything (except maybe a minute or two of free time).
Many people think that buying a faster CPU makes it more "future-proof". Essentially, this is wrong. If an i5 chokes at some future application (that is, it is unable to process the code in a reasonable time frame), so will the fastest i7 - in the end, the performance difference between the two is less then 15%, and thats under maximal throughput. For tasks which do not tax the CPU, there will be no speed difference at all.