Get confused about the keyframe strategy containing **low features**

[RonghaiHe]

I get confused about the keyframe strategy that if the number of features is less than the threhold: min_number_features, then this frame would be considered as a keyframe.
But why? Doesn't a keyframe represent a certain scene? low features would be appeared in some special conditions, like the motion blurs. Could this frame be a keyframe?
I have also read the paper Kimera2 and it doesn't mention such strategy.
Hope to obtain an answer. Thanks!