"This is what the Lord has commanded, ‘Gather of it every man as much as he should eat.." The sons of Israel did so...he who had gathered much had no excess, and he who had gathered little had no lack; every man gathered as much as he should eat.
Moses said to them, “Let no man leave any of it until morning.”
(Exodus 16:15-21 NASB)
The manna would melt away with the heat of the day, so that each night it looked like there was nothing for the next day.
Each night they had to believe God would provide all they needed for the next day.
By leaving some until morning, they were putting their security in the provision of God rather than in God Himself as their Provider.
What is amazing is that manna had not existed on earth before God provided it for His people, and when they reached the produce of Canaan, the manna stopped. So, not only was the manna provided, but the worms for the "leftover" manna were also provided!
(And evidently, these were Sabbath-keeping worms, because they didn't eat Sabbath-stored manna!)
The worms were purposefully sent to teach the Israelites not to trust in their own clever scheme to meet their needs apart from dependence on God.
"Do not worry then, saying, 'What will we eat?'...for your heavenly Father knows that you need all these things. But seek first His Kingdom and His righteousness, and all these things will be added to you.
So DO NOT WORRY ABOUT TOMORROW, for tomorrow will care for itself. Each day has enough trouble of its own."
And each day has enough manna for that day from the Provider that lovingly sends worms to the things we've hoarded, to keep us from trusting His provision instead of Himself.