Zero offset seatposts make zero sense, mostly. Here’s the exception: bikes with really slack seat angles. 71, 70, 69 degrees. These bikes can use zero set back posts. The soon-to-be-available Tanglefoot bikes are designed around said posts. We like the look of zero offset posts, but most folks put them on for the wrong reasons. Here’s some of them:
To shorten reach to the bars. BAD! Bar reach should be addressed with a stem and your bars, not your leg’s relation to your pedals. Moving your saddle forward is a bad way to adjust reach. It’s the worst way. It is a great way to put more strain on your knees, though.
It’s what they had laying around. Most bikes come stock with 0 posts, because 0 offset posts are typically cheaper to make than 20mm offset posts. People don’t notice knee pain on parking lot rides. True facts. Using a 0mm post 'cause it’s just what ya got is like subbing in Gak for eggs in an omelet recipe, cause it’s just what ya got. Go to the damn store.
0mm posts are lighter. You can make a really light 0mm post, maybe 50 grams lighter than a set back post. 50 grams is not worth knee pain. Subtract 50 grams somewhere else. Get Ti bottle cages. This is not the place to do it.
If you have a slack frame, get this Nitto post. It’s the best 0mm post there is. Big bolts, not under built, nice looking. Off road safe. Won’t slip. No gimmicks.