Once a network is congested, various service protocols (statistically
insignificant in terms of additional network load, but mission-critical)
can’t do their jobs. Here are some examples
DNS - adding hundreds of ms of latencies to turning a website into
an IP address is not good. With a typical web page doing dozens,
even hundreds of DNS lookups, DNS not getting through in a timely
fashion results in vastly slower browsing.
NTP - the network time protocol - relies on somewhat timely delivery
of packets in order to keep your computer’s clock accurate. Lots of
things rely on accurate timekeeping.
ARP - the address resolution protocol - also relies on timely
resolution in order to even find other devices on your network.
DHCP - if these packets are lost or excessively delayed, machines
can’t get on the network in the first place.
Routing - many routing protocols depend on packet drops as a way of
monitoring network health and are time sensitive.
VOIP - needs about a single packet per 10ms flow in order to be
good, and less than 30ms jitter.
Gamers will get fragged a lot more often with latencies above their
IPv6 relies on even more specialized packet types for
autoconfiguration, e.g. the equivalent of ARP
Encapsulated packets (VPNs, X11 over ssh, IPv6 over 6rd/6to4)