Let's say you are writing into a byte[] and have a LEB128 length-prefix followed by a payload, but that determining the length actually involves nontrivial encoding work. For example, you have a UTF16 string and want to write out a UTF8 string, you want to go over the characters and write them out, but the UTF8 length is not known without doing all of that work.

If you can choose a fixed number of bytes for the length prefix, you can skip that number, do the encoding and find out the length, and then come back and fill in the length-prefix after.

But you actually don't know how many bytes it will take without doing all of the work to know the payload length (since larger payloads take more bytes to represent the length).

If you allow overlong representation you can reserve a few bytes and sometimes it'll just be the effective no-op bytes. If you don't, you won't be able to.