You stop spending hours on links that pass nothing.
Some listings pass search authority to your site and some do not. Knowing which is which decides where the hours go.
A dofollow link passes ranking signal. A nofollow link tells search engines not to pass it. Both can send you human traffic; only one helps you rank.
Open an existing listing, inspect the outbound link, and look for rel="nofollow" or rel="ugc". The directory listing in SubmitMap records this where known.
A high-traffic nofollow listing is still worth the twenty minutes. A dead dofollow directory is not.
Some sites strip the attribute for paid tiers only, or change policy. What was dofollow last year may not be now.