Assets are used to keep track of things. You could use it to keep track of the stocks you own, for example. I also added my car to be able to assosiate costs with it.
You cannot manually set an amount or value per unit. You can only record these values at a specific timestamp. There are two forms that let you do this. The Buy/Sell with transaction form automatically records a transaction. The additional cost is for recording costs or fees assosiated with the transaction that don't factor into the value of the asset. You can override the Total if you need to as well. Alternatively you can use the Update without transaction form. I like to use it when updating share prices.