So when you use SNAT Automap in the Policy, the client will get an IP address from the DHCP range that you have configured in the Network Access settings. However, SNAT is applied to client traffic when it goes through APM and onto your network - this is normally to ensure the return path.
If your routing is such that the DHCP APM range will route back to the APM then you don't need to enable SNAT. This way, when you look at the traffic you will see the source IP as being the DHCP address rather than the F5.
Further reading can be found here