Application memory ignores non-reducible cache memory. So the widget might show that you have extra several gigabytes, but actually there is no more available memory, and thus may give you false impression and result in OOM situations. For example:

(widget here is patched and displays allocated memory instead of application memory)

On this image you can see that Application memory is 10 Gb. You might feel that you have 5 Gb free and you're safe. But actually, you have less than 2 Gb available.

One of the reasons of why it happens is tmpfs systems (such as /dev/shm/) and Chromium engine that uses it for sandboxing and takes considerable amount of space in cache and this space cannot be freed up by the system, and thus there is quite considerable amount in pagecache cache area that could not be freed up if needed.