If something is spendable, it means you can use it to buy things.
Spendable things are good because you can use them to get what you want.
Examples:
I have some spendable money.
She gave me spendable coins.
He has a lot of spendable cash.
I wish I had more spendable money to buy toys.
In the game, you can earn spendable points to get new things.
I need more spendable money to buy a new toy.