The at command lets you queue a one-off task to run once at a specific time, so you don’t have to write and then remember to delete a cron entry for something you only need to run once.
You’ve probably used cron to schedule tasks and then realized the job only needs to run once. A common example is giving a vendor temporary SSH access to a production server for troubleshooting. You might open the firewall for their IP address and plan to remove the rule a few hours later.
The problem is that it’s easy to forget. Production issues happen, priorities change, and before you know it, that temporary firewall rule is still sitting there long after it should have been removed.