The customizer is a place where you can preview changes to your site before saving it. Most theme authors work their themes to look good in any devices. Previewing site changes is just important on mobile as it is on desktop, to ensure that front-end user experience is exactly as intended.
A propose of this feature has been made to bring this idea together. The proposed feature focuses on simplicity. The device previewer is in the customizer controls footer, near the “Collapse” button. Only three options are available by default, and they’re intentionally ambiguous. Rather than looking like specific devices, the intent is to understand what a site may look like on a roughly tablet-sized, portrait-orientation device or a roughly phone-sized device, in addition to the standard desktop view. A further extension may allow larger screen sizes to be previewed on smaller devices with forced horizontal scrolling; for now, the feature is only available on larger devices.
Although there is no advice yet that this will be added in the next major release of WordPress, but hopefully this will be added in the next patch release. You can track updates or comment here.
For developer like me, this proposed feature is a great functionality.