PER were defined in 1994; back then applying zlib wasn't something you "just" do. Modern use is backwards compatibility (or cargo cult.)

UPER is extremely compact encoding format. It still makes sense to use UPER, because after all, it is an international standard and telecommunication protocols itself are supposed to add as little overhead on top of actual payload as possible.

For example, if you have ASN.1 UTF-8 string that is constrained to 52 specific characters - UPER encoding can present every character with 6 bits (not bytes).

In modern world you can apply zlib on top of UPER encoding or internal payload, however, depending on the use case.