To add weight to the power bank
If you’ve ever purchased a power bank, you’ve probably noticed that it makes a weird sound whenever you shake it. Companies will tell you not to shake them to prevent destroying the inner components. The OP was curious what was inside of his power bank, so he took it apart to see what was inside.
Apart from the regular stuff (a rechargeable battery connected to the charging port), he found out the source of the weird sound. There was a capsule filled with sand that apparently served no purpose. It was there to add a bit of bulk to the device. Clever.