Most of the time, data binding IS one-way when using Knockout. Input field? That's one-way from display => a variable in your viewmodel. P tag? That's one-way

Most of the time, data binding IS one-way when using Knockout. Input field? That's one-way from display => a variable in your viewmodel. P tag? That's one-way from variable in viewmodel => display. But when do you really need two-way data binding? Form validation and formatting. Man, this stuff is hard even with two-way data binding. You have to be able to accept, parse, format, and validate all in the same element. It's very helpful to be able to use knockout for a task like this.