there might be things in your photos that you wouldnt think of adding that others might. like somebody might want to know whether the crosswalks have tactile pavement, somebody else might want to add cycle lanes, bus stop numbers or the position of electricity poles. when you think about each person having to go around their town/city each time they want to add certain things, its much more efficient to have 1 person take photos and then have other people reference them
even just for myself, there were always things i would forget to check when i was in town, like did a bus stop have a bench or a bin etc. and there would be other things i would see on my way that i would end up forgetting by the time i got home so thats mainly why i started using mapillary
the OSC map is empty where i live as well. so was mapillary when i started but there seemed to be more of a community around mapillary, it had a more advanced app and website, and i liked that is was more of a neutral company compared to OSC… which is why i chose mapillary in the end