Basically you're introducing a hole. For example, if you have some devices in your network (like a dodgy TV box) that are not supposed to reach the internet or other parts of the network, the computer with net.ipv4.ip_forward=1 could be used as a pivot. Depending on the routing tables you probably would also need to enable IP masquerading (NAT) to allow bidirectional communication.
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Basically you're introducing a hole. For example, if you have some devices in your network (like a dodgy TV box) that are not supposed to reach the internet or other parts of the network, the computer with net.ipv4.ip_forward=1 could be used as a pivot. Depending on the routing tables you probably would also need to enable IP masquerading (NAT) to allow bidirectional communication.