The VFB is much more than a render image holder window. The new enhanced version provides a higher level of post processing adjustments and ability to modify your scene from inside the VFB. Here is a list of old and new benefits you should know:
Image preview: Loads render elements channels via a dropdown list; Works with display corrections; Loads various 8bit and 32bit file formats, including V-Ray resumable files; Image comparison between 2 or 4 renders;
Image management: Keeps the rendered image in full 32-bit floating point format; Can store history of rendered images with their post rendering corrections; Can save corrected output to various image formats;
Image editing and post processing: Set Color Corrections to rendered image and its elements; Composite mode for Back to Beauty workflow including layers corrections; LightMix control on lights contribution in the rendered image; Applying Proportion Guides on the rendered image;
Scene editing: Can modify actual lights values by transferring updated intensity and color from LightMix mode; IPR camera navigation and focus control from inside VFB;
Render management: Denoiser control; Region rendering control; Allows you to choose the order in which the buckets are rendered; Automatic render resolution adjustment in IPR when the VFB window is resized;
To use the VFB for rendering, create a V-Ray Renderer node.
VFB also has some limitations which are listed in the Notes section section below.
