A party round spreads a round across numerous angels or micro-funds, each writing a small cheque, rather than concentrating it in one lead investor. It can close quickly and bring many helpful contacts onto the cap table.
The trade-off is the absence of a committed lead: no single investor negotiates terms rigorously, takes a board seat, or reliably leads the next round. Heavy cap-table fragmentation can also complicate later governance, which founders often manage through a syndicate or SPV.