Templates allows you to create invoices that are commonly used quickly. You may have a combination of items that are sell together with an special discount, then a template is the best way to create invoices.
Search and select multiple items, specify quantity. Price cannot be changed, during the creation of the invoice, the price is taken from the item. Use charges for promotions or discounts. If you have agree and price with your customer then you can use contracts.
Charges allows you to setup surcharges or discounts for a template.
Add taxes only in the case that the current template needs an special tax treatment. Otherwise when creating invoices from template the taxes are defaulted from the company taxes.