What happens with MASQUERADE and packets that origin on the host itself?

Given the following network:

                     +-- endpoint 1
                     |
internet -- server --+-- endpoint 2
                     |
                     +-- endpoint 3

where the endpoints are on subnet 192.168.1.0/24 and they route their traffic through the server.

For this, we require a NAT rule on the server for the interface connected to the internet:

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE 

Two questions:

1. Packets originating on the server itself are also matched by this rule. They already have the correct source IP. What happens with them? (Experimentation shows it works.)
2. I always considered the question to add MASQUERADE or not to be a property of the interface (because the connected network can either handle multiple source IPs or it needs a NAT). Is this a good mental model?