From the module description:
Use Views Global Filter on sites that feature multiple Views. You want the views to be driven by the same filter selection, for instance a country, a date range, a search term or even proximity to a post code. These views may be on the same page or scattered across several pages on your site, however you want to offer the visitor the convenience of setting the filter for all these views only once, for instance when they first land on your site.
Views Global Filter implements the picked once, operates site-wide (POOS) principle. This means you can place anywhere on your site a selection widget (drop-down, check boxes etc) to globally filter all or some of your views. When the user picks a value from the global selector, the value is invisibly passed as a contextual filter to whatever views you like, even when these reside across multiple pages. This avoids you having to duplicate filters and avoids the visitor having to re-select their choice as they go from page to page.
This module is probably useful for sites that want to implement various global preferences that apply throughout a site. However, its effectiveness only applies to Views.
