A pruned node discards from persistent storage (disk, SSD) blocks for which it has no further use.
A pruned node therefore cannot help synchronise a new full-node
A pruned node can pass on unconfirmed transactions in its mempool and, at least in principle, any blocks it retains in memory or cached in other temporary storage.
A pruned node downloads exactly the same amount of data per month as a fully-synchronised full-node. It probably uploads less since it retains less. Actual amounts probably depend on connectivity and levels of Bitcoin network activity (i.e I haven't measured this).