Why do some English sports clubs have the word 'United' in their names?

I am wondering what is the reason for doing so. I do know that Plymouth United F.C. was possibly the first sports club to do so, but I couldn't get any details on why they chose to include the word 'United' in the official club name. Any ideas? And it just seems that only English teams have 'United' in their names although the MLS also has teams that have used it in their names.