It allows you to fill in padding in a buffer. For example, all data in a buffer will be interpreted by a downstream system, and someone pre-calculated the size of that buffer. Rather than encode everything twice (once to figure out the exact size needed, and a second time to actually populate the buffer) the buffer size was calculated using foreknowledge of how many values would be written to that buffer and then just pessimistically assuming all of them are max-size so writing will never fail. Another situation is when you're rewriting part of an already-encoded file. If you want to change a bit of payload then using padding bytes gives you more flexibility so you can do that without having to do any memcpy into a new buffer.

It's uncommon but I've definitely seen it done (with media containers like Matroska, not actually LEB128) in extremely high-throughput systems that can't spare any cycles.