There are several graphical PCB viewers available and they read what appears to be a standard board file (.BRD) format. Eagle and other PCB designing applications generate board files, but also generate Gerber files. What’s the difference between the two formats? Is there a reason why you would choose one over the other? Do board files contain only a subset of the information contained in Gerber files for IP protection, performance etc.?
Board files are effective "source" information specific to a particular design package and suitable for editing. Elements would have associated metadata indicating their place in the design, for example a net name or number.
In contrast, Gerber files are a portable output, intended only for fabrication and not simply edited, at least in more than trivial ways. Gerbers would normally contain no metadata about the purpose or identity of any graphical element.
A comparison of purpose could be made by considering a spreadsheet file compared to a PDF file capturing a printout thereof.
In addition to @ChrisStratton 's answer, I'll add the case of when you "use" one over the other.
Gerber files adhere to a standard, brd files do not. Although board houses might accept brd files from a number of different applications for production, there is a non-standardized step between turning the board files into boards, and there is no such step with Gerbers. I recommend submitting Gerbers to board houses.
*.BRD files are XML formatted to define the board in 2D or 3D for viewing.
Not preferred for production.
Gerber files are just the G codes for optical paths for copper traces for PCB production without labels like Vias.
A global G-code table defines each trace width or router for slots and drills for holes.
Both files are IP. Neither contain logic diagrams.