There are several benefits of using a directory submission service. Primarily, you can save time and money by using a submission service.
Then you would have to go through the submission process for each directory which includes following every directory's guidelines, choosing appropriate categories and subcategories, filling up each submission form, alternating between different titles and descriptions, entering the verification codes, etc.
This process could easily take up to 5 minutes per directory, hence over 8 working hours to submit to just 100 directories. Apart from this, you would also have to ensure that you keep track of each submission made, to avoid duplicate entries.
Instead, it would be much more economical and timesaving to make use of a directory submission service. To submit to 800 directories, you would be essentially saving approximately 67 hours of work, besides the hassle of maintaining and constantly updating your list of directories to include newer directories that match your original criteria.