In a situation where everyone is negotiating in good faith, when a company offers you full time salaried employment they're expecting you to stick around for awhile. Depending on the company, this could be a year, two years, ten years. They don't expect to terminate your employment themselves. That's the commitment tptacek is speaking of. They're committing to buying all of your available work hours for an semi-defined period of time in exchange for a regular paycheck. Their legal commitment to you is generally extremely limited, but also not relevant to this discussion.
In consulting/contracting/freelancing terms, if a client accepts a multi-year retainer with a large number of hours, you'll likely give them a bit of a discount relative to your normal everyday rate. They're committing to continuing your business relationship for the duration of the contract, even though the contract will almost always state that either side can end it at any time with a certain amount of notice (just like full time salaried employment).
In consulting/contracting/freelancing terms, if a client accepts a multi-year retainer with a large number of hours, you'll likely give them a bit of a discount relative to your normal everyday rate. They're committing to continuing your business relationship for the duration of the contract, even though the contract will almost always state that either side can end it at any time with a certain amount of notice (just like full time salaried employment).