A client responsibilities clause lists the specific things the client must provide — on time and in usable form — for the project to proceed as contracted. It creates contractual obligations on the client side, not just the provider side, and specifies what happens when those obligations aren't met: timeline extends, not your fault.
Plain English definition
The clause says: the provider will deliver on the agreed timeline IF the client provides [list of inputs] on the agreed schedule. If the client doesn't provide those inputs on time, deadlines shift by the same amount, and the provider isn't in breach for the resulting delay.
When you need it
Any project where you need something from the client to proceed: content, brand assets, system access, approvals, decisions, sign-offs, subject matter expertise, or review feedback. In short: every project. The provider is never the only active party in a project.
What happens without it
Without a client responsibilities clause, your timeline obligations are unconditional. If the client doesn't provide what you need, you're still expected to hit the deadline. When you miss it because of their delay, you're in technical breach — with no contractual protection.