I was advised by a friend (who's a published author) that I should pay to get MS professional edited before I send it to literary agents but A) I can't afford that and B) I have never heard of having to do that when going through traditional publishing
I am a writer of Historical Fiction and I finished my first book of the planned trilogy after 8 years of writing and 20 years of research. All that counts for naught if it is not professionally edited.
I found a great "Line Editor" and we are going through my book chapter by chapter. Finding a good editor is very important, because cooperation is vital. Moreover, English is not my first language and this makes such editing even more important.
My manuscript is finally becoming a book.
Invest in such editing. Take it step by step and for the love of Universe, avoid "Contribution Based Contrast". They are formulated in such a way that you get profoundly screwed over.