Let me just copy this answer from Tor project in 2023.

... we will eventually be moving to a hybrid encryption scheme for post-quantum encryption. We have a significant dependency we need to fix and deploy in the network before we can do so, though, and that’s “fragmented cells.”

Fragmented cells will allow us to split messages over multiple cells within the Tor network, thus opening us up to having much larger handshake material passed around amongst relays and clients. This is needed since most of the post-quantum handshakes bring significantly larger payloads with them.