If you are a local plumber then the chances are you are never going to rank on google in the top 10 under the search phrase plumber.
However as a plumber you may specialise in providing walk in showers for the elderly. Walk in showers for elderly people is a niche so therefore keywords would be targeted around that. This would produce better search results then generic search terms.
If you have case studies or images available on the website then name the files that match what you expect people to search for. For example the plumber above may have a case study about a walk in shower installtion therefore the file would be named appropriately.