Go into YouTube Studios, then click 'settings', then go to 'channel', and then click on 'feature eligibility'. There is a link called 'status and features'. When you click on it you can see all the features you've unlocked and which you can still unlock and how to do so. At least that's how it works on PC.
One feature that you unlock with 100 subs is a custom channel id. This allows you to use a link like youtube.com/channel/c/username instead of youtube.com/channel/[UCH3hO838NbfQ_KgKiSFLKfw].