A billing account is the cloud account that pays for the resources used by one or more projects. Charges from each linked project are collected on it, and it holds the payment method and invoices. In many clouds a single billing account can be attached to several projects, and a project must be linked to one to use paid services.
This matters for free tiers and credits. They are often applied at the billing-account level, so several projects linked to the same account can share one allowance rather than each receiving its own. Creating extra projects therefore does not necessarily multiply what you get for free, and usage in one project can use up credits that another project was counting on.
How this works depends on the provider and the specific offer, so the terms of a given free tier or credit should be read directly. Some offers are limited by region, product, time or the number of accounts allowed.
To avoid surprises, check which billing account each project is linked to, review the terms of any free tier or credit, and set budgets and alerts so that unexpected usage is flagged early. Review the linked projects periodically and remove any that are no longer needed.