Thank you fkalich for sharing.
I can post some considerations.
I use the 7/8 time signature if I need to "cut" the last beat in a standard 4/4 piece.
Click to view attachmentI can use also the same 7/8 time signature if I write a riff which fills the measure in the right way.
Click to view attachmentAnyway, it's all related to the "speed" of the note value you use as beat.
Also, for odd time signatures you can count beats in different ways.
7 can be thought 1-2-3-1-2-3-4 or 1-2-3-1-2-3-4 and so on.
A famous song which uses the 7/4 time signature is
Money from Pink Floyd.
Before the metronome, musicians used words and time signatures to explain the tempo.
Today, fixing a bpm and a reference note as beat, there is no more ambiguity.