Well, for example, say you were adding a new preference page. First, I'd do a deploy of the new database table(s), even though they aren't used. Then, I'd add a link to the page in the UI and put that behind a flag, maybe with a small non-functional UI so that the designer could play with it. Then, I'd implement the basic functionality and open it up to my team to start playing with it. Then, I'd ship other deploys for things like tests, more edge cases, UI tweaks, etc.

At some point in there, I'd open that page up to my beta customers, trial customers, or whoever is less risk-adverse. Once I'm happy with it, I'd do a percentage rollout to ensure any issues don't affect all customers at once. Just an example, but the idea is to reduce risk of each step and make each step easily reversable/hidden if something comes up.

It's not, and it cannot ben deployed in increments.

What OP and many people are missing is that a lot of us don't work on cool and hip new apps. Some people aren't in a osition to push incremental changes to a payroll system.