Because it's transparently compressed, so you get more RAM out of your RAM at the cost of some latency and CPU usage. This is a good tradeoff on heavily RAM constrained workloads like attempting to run Slack clients.
"Zswap is a kernel feature that provides a compressed RAM cache for swap pages. Pages which would otherwise be swapped out to disk are instead compressed and stored into a memory pool in RAM. Once the pool is full or the RAM is exhausted, the least recently used (LRU) page is decompressed and written to disk, as if it had not been intercepted." https://wiki.archlinux.org/index.php/Zswap
If think of you added in an SSD to the machine, then zswap might be more appropriate.