I wanted to reply to someone recently by just saying Thanks!. I tried to post and it got rejected for the minimum character limit. I know I could have just used or a like, but I prefer to say thanks.
This is a default that has been set on the system to deter spam bots. Although not the most helpful when you want to reply with a simple thank you. Having this setting allows the system to detect a spam bot more easily as they have trouble creating longer and meaningful sentences.
It may also be useful to know that a message has a minimum of 10 characters and a DM (chat) has none. It might not be ideal, but this could be used as another way of thanking people personally.