To know the customers better is the most factor before going for keyword research because If you're not clear about the product how can you shortlist keywords for it. If there's no clarity about the product, road of seo plan would go in mess and give bad results. Always be aware of the range of products your customer have and built your strategy accordingly...
Main purpose of doing SEO is the earn relevant Customer. In seo, Keywords are playing vital role to reach at right customers which will eventually help to achieve desire result. If you make mistake in identifying right customer which leads to wrong keyword research. you will never able to achieve desire result even if you could rank well for those wrong keywords.
You need to know your customers in order to know what subset of possible keywords they are most likely to use.
I am in the music business - Possble keywords include concert, club, gig, dance, band, singer, songwriter, song, musician
I know that my visitors want to go to concerts and clubs rather than book bands or singers for functions. I then need to be sure that possible synonyms are mentioned in a natural manner. My traffic picked up significantly when I realised how many people were using "gig" as a search term and did some minor re-writing.