There are a number of useful things you can do with the rubies you earn by maintaining your streak and studying every day.
You can donate rubies to reward people who make useful comments, or use them to unlock special features in your courses.
In the ruby store you can buy:
Freeze: Streak Protect, which will make sure that you don't lose your streak when you forget to study for a day. Make sure to get another one after you use it!
Double or Nothing, which wagers 7 rubies on maintaining your streak for 7 days. With this, you can spend 7 rubies to earn 14, as long as you manage to keep your streak.
Fast Forward Time to make variations in a chapter or course reviewable immediately. This is useful for tournament preparation.
Magic Streak Restore, which allows you to restore your highest streak. Keep in mind that the cost will depend on your streak.
Username Change Token, which allows you to change your Chessable username.
Plus One: Free Course Slot, which increases the amount of free courses you can get. Spend 100 rubies to receive an additional slot!
You can learn more about rubies in this video.