The one type of die that really needs a magnet storage sheet is alphabet dies. The magnet sheet helps you keep track of each die so you aren’t left with a less-than-26 letter set.
I have had to dig through the recycle bin more than once looking for a die that got mixed in with the paper scraps, but without a magnet sheet I would not have realized it was missing until too late!
There are several purpose-made craft storage solutions, including Lawn Fawn, Totally Tiffany, Sizzix, Hero Arts, Crafter’s Companion (and more I’m sure), but plain magnet sheets are also readily available. The one pictured is this one https://amzn.to/3Op6oRl that runs less than a dollar per 5×7 sheet. I usually pair them with DVD storage sleeves for a bargain approach.
Papercraft Crystal 