For example, winterizing your home. You only do that once a year and there are lot of little steps and places to check. Rather then having to discover them all over again just learn them once and write it down. Several times at work I created or helped people create checklists or procedure files to remind people how to run this or that monthly report.
A really good thing about making a checklist is you can use it to find places to streamline, automate or outsource. At my last place of work we had to combine several piles of information into spreadsheets so we could standardize the information before putting it into a database. By making a checklist of things that needed to be done we could break things up into a assembly line, each person did a few things and passing it on. I also found a few ways to script, macro or otherwise automate an action. We went from 500 rows per day to over 4000.
It works in other ways as well. A first aid course will teach you a checklist of ABC: airway, breathing and circulation. They make it easy to remember the most important things to do which you will only do a few times in your whole life, unless you're a first responder.
You can store 3 things in long term memory pretty well, but it under stress that is about as far as short term memory can handle with everything going on.
If it is anything more then about 7 items it is best to write it down and just use a real checklist.