Many times when I started search engine optimisation of websites I proclaimed I didn't need to undertake any keyword research, as I already knew which keywords the customers would be using. It was just so obvious which keywords they would be using. Then I did the keyword research anyway, and found out I was miles off.
Everyone expresses how they think in different ways, and this is never more apparent than how people type in searches, looking for the same websites, but with completely different keywords. Some people would look for cheap flights to France by typing in "Cheap Flights to France", some would type in "I want to fly to France for cheap", and others would type in dozens of other ways of expressing the same needs. Which keywords and phrases do you optimise your website for? All of them. But without the research, you'll have no idea how many you're missing.
The old saying "If you fail to prepare, be prepared to fail" is never more true than with Search Engine Optimisation. If you don't know what you are aiming for, you'll have no way of achieving your goals.