Confusables are a common problem that editors find in texts. These are words that may sound (or almost sound) the same, but they are spelt differently and mean different things. They could be words like: Site/cite/sight, their/there/they’re, to/too/two, passed/past, effect/affect.
There are also confusing words that have meanings that are very close, but they need to be used with care. These words may have a similar root, but they do have different meanings. They can need careful thought: Disinterested/uninterested, ensure/assure, comprise/compose, infer/imply.
Even the most basic PC is loaded up with editing software: the spellcheck with its spelling and grammar checks, a thesaurus and even links to online dictionaries for translations. With all this material at your fingertips why do you need an editing service?
Some professional writers may feel their language skills are exemplary and they don't need someone else to check their work. When you have all this material at your fingertips you may wonder why you would need a professional editing service to check over your work.