First of all, it is always possible to use a payment terminal "manually". This means that, technically, in the PoS app, you will have a payment method for the bank linked to that payment terminal but the terminal will not be linked to the PoS app. By doing so, you will be able to select that payment method at the order confirmation and then manually type the amount on your terminal in order to, in the end, manually validate the payment in Odoo.

But when is a connected payment terminal interesting? 

• It is almost required when you have plenty of transactions in your shop. It's mostly a game changer when you have to process a lot of payments. It is also less prone to errors.
• When you already work with one of the payment terminals that are integrated with Odoo such as the ones that are presented on this page Website
• You do not use any payment terminal already but you plan to do so and you want to integrate one that is proposed in the link above.