Hello, I'd like to help you with your question.
I am so sorry for the loss you suffered with your husband's passing. The grief from losing him so suddenly plus having to sell your home would be difficult. I hope you have all the support you need to help you work through the grief.
The reasons why someone would save money in a box in a garage varies a bit. If the person has no money or at least appears to have no money, they may hoard money so they feel safer. If your husband ever went through a time in his life that he had nothing, then that might explain why he saved this money. Once a person suffers through the fear of having nothing, they sometimes have a hard time feeling safe even when they have a lot of income so they save "just in case". It is just a way to reduce their anxiety
Another possible reason could be that your husband didn't completely trust banks or other financial institutions. Even if he had most of his money in a bank, he still could have had issues with completely trusting that all was ok. So he saved money and hid it- putting it in a place where no one would guess it would be. He may have eventually forgotten about it.
He also could have saved the money from some special situation in his life. Sometimes people will associate gifts from others or money from someone who has passed on as a link to that person. So instead of considering the value of the money, he might have associated the money with the person, just as you might with a book someone special gives you with an inscription in it.
Finally, the money might have special meaning through the type of money it is. It could have collector value for some reason. Many people collect certain bills for the rarity of them or other similar reasons. You may want to have your bank or a collector take a look to see if the money has any particular meaning.
I hope this has helped you,