You don't need to route all traffic through the Raspberry, but I'd also not use a "dumb" hub, since a) these are hard to find nowadays and b) they multiply the traffic on your network.
There are managed Ethernet switches which can forward all traffic to a monitoring port and the Raspberry can then capture the traffic on the monitoring port. At the same time, the switch will still forward the packets on the correct port, so it'll have less impact on the network than routing everything through the Raspberry.
I have good experiences with a HP-1810G (Gigabit), monitoring with my laptop. It is at ~80 € for the 8 port version. For the Raspberry, you could use a Gigabit USB network adapter (which will be faster than 100 MBit) or just decide for a cheaper 100 MBit version of the switch.