I was a freelancer and it was my first gig in the industry. The parent company eventually fired him and in doing so, everyone lost their jobs. True story.
If you don't answer a Slack message immediately, how to you remember to answer it later? It's not like email where you can archive threads that are done. Instead chats (i.e. channels, private messages or threads) with unanswered questions look exactly like other chats.
This assumes some messages are urgent and some are not, and that you can't know which is which until you open the chat and thus mark it as read.
You either mark it as unread if you are going to get to it as soon as you wrap you what you're currently doing, or you click "More actions" and select a "Remind me about this" option.
"Mark as unread" just makes it look like an unread message. I want to read every message as it comes in, in case it's urgent. "Mark as unread" would either obscure new incoming messages from the same sender, or cause me to unnecessarily triage the same message several times.
"Remind me" is time based, which is too arbitrary. I want to instantly see everything unanswered, not have it trickle in at random intervals. Using the "Save" button as "Move to inbox" as nthj suggested makes more sense. It's also easier to click.
Slack has saved messages. I use this as an inbox/todo list. Any message that takes longer than two minutes to reply to, I move to my actual todo list and un-save the message.
At my company, we send messages when we need help, but the expectation is that the reply comes at the convenience of the person being asked.