APIs decide how easily your systems can be connected. When NetSuite, Salesforce or Microsoft 365 exposes an API, an integration can update records directly instead of relying on someone copying data between screens. Checking API access early is one of the fastest ways to judge whether an automation idea is practical.
For example, an order portal can use the ERP's API to check stock before confirming a sale. A common limit is that having an API does not mean it exposes what you need. Some APIs omit key fields, cap how many requests you can make or require a higher license tier, so confirm the details before committing to a design.