What in particular are you concerned about protecting feet from?

DD2 still rarely ever wears shoes and she's 19mo (been walking since 10months too). My mum keeps on at me about it so I have been feeling a bit slack but I don't exactly understand the concern either. So far we have had no issues and we are down at the park most days. To be honest I'm more likely to put her in shoes for a trip to a restaurant than the park lol. Not to derail the thread but is there something I should be worried about?