For me it's dependencies and automatic urgency ordering. Greys out whatever I can't do yet without actually hiding it, helps decide what's most important to do next, and automatically adjusts all of it when a task is completed.

I did only use a text file before fiddling with those two features and finding they did actually help me. Nowadays I use taskwarrior for anything that will last longer than a day, and a temporary text file for only the things I plan to do that day.