I don't use SnapChat, but once you have children, your parents and siblings are interested in seeing the funny little day to day things your kids do. 5 seconds here and there of eating breakfast, getting dressed for the day, running around, etc. Twitter and Instagram don't really cover that use case well at all.

I use private photos/videos on Instagram to share those things with my family, but it's very clunky and would feel like spamming to send more than 2-3 curated photos per day. I gather that with SnapChat there is far less curation.