Adding Collateral is useful in two scenarios. The first is when a Vault is coming close to being liquidated (link to liquidation), adding collateral can save the Vault. The second is when a Vault owner wants to increase their capacity for generating pUSD, adding collateral will increase the maximum amount of pUSD that can be generated.
To add collateral, click on "Deposit" and fill in the intended amount on the Deposit page. And then enter the quantity of collateral you want to add and click Deposit in picture on the right
Pay 4 MOB token and see Details, the collateral Increase from 96.8865 to 100.8865. You can see details about this adding in chat window.