Take a simpler example: what would a time signature of 2/3 mean?
If you divide a whole note into three equal parts, in conventional notation you would write that as a triplet of half-notes (= UK minims). But suppose you want the music contain some normal-length half-notes followed by just two notes at the speed of a triplet, and them continue normal-length notes.
You could mark one bar to be played at a faster tempo (MM mark) and then revert to the original tempo, but the performer would then have to "do the math" to figure out what was intended. If the tempo marks were MM 76 and 114 it's not obvious to most people that the ratio is exactly 3:2.
You could write a series of triplets with notes tied from one to the next, but that would be unreadable except for a short passage of music.
The new notation of a 2/3 time signature was invented as a neater way to write this. It's easier to "read" the exact relation between time signatures like 2/3 and 2/4 than between MM marks of say 76 and 114.
Some composers have used a different notation, where the denominator remains a power of two (showing the length of the beats) but the numerator is itself a fraction - for example (5/3)/4 would be a bar length of a quarter-note plus two-thirds of a triplet of 8th-notes. Of course you could write that more conventionally as 3+2+2/8 or 7/8, but hey, avant garde music needs to look cool, and 7/8 is way too old school for that.