It was an option to originally help people running lower FPS when the time spent in each frame is artificially high due to frame limiting software.
The best way to explain it is with this example:
PC 1 - 60 fps = 16.6ms spent per frame
PC 2 - 10 fps = 100ms spent per frame
On PC 1, when the frame is left, at minimal, 16.6ms will elapse since that's how fast the client is rendering. This means there won't be any noticeable EB execution difference.
However, on PC 2, at least 100ms will elapse. If EB has to leave and enter the frame multiple times (which it does) then you start to see really large execution delays in EB because of how slow the game is rendering.
For PC 2, By not leaving the frame, the problem is avoided because all operations are done in a single frame, which lasts about 100ms, so there's more than enough time to do stuff.
However, on PC 1, if you were to do that, then any operations that take more than 16.6ms will cause frame spikes and sutter issues, and lower the frame rate because more work is being done per frame than the target framerate allows, so the client is negatively affected.
