is it useful to submit software to download sites?

The best way to promote your software? Opensource! Put it github and build a community around it.

actually submit software to download sites can really do good for the software promotion.
after you submit your software to download sites, many potential users will see your product.
Some software vendor also chooses software auto submitter to submit software to download sites to save huge amount of time. Another important reason is the software has many valuable download sites in it which saves you much time to find the download sites everywhere.
Here is one perfect auto submitter: http://www.submitworker.com/