One of the main attractions of using AWS is its pay-as-you-go model. You only pay for what you use, no more and no less. Unfortunately, this can sometimes result in what's known as bill shock at the end of the month. This happens when you do something that you might not know is a charged service, or you do not know how much is charged for it, and you don't find out until it's too late. Especially when getting started, users may not fully appreciate the cost of the activities they're undertaking.
Creating a budget and setting up alerts that get sent when you exceed that budget is good practice for all AWS accounts. For a development account, where you are only testing things out for short periods of time, you may never expect to exceed the free tier, so you can set a $1 budget and get alerted if charges exceed that amount, and then quickly find...