mentioned, "switchport mode access" is a way to force a switch port to always behave as an access port. When the switch port behaves as an access port, it is pretty much acting like consumer-grade switch with no capability of establishing trunks.
From operation and security perspectives, you may want to set certain switch ports as always access port. You don't really want some unknown switch or unsuspecting devices, introduced by some clueless users, to suddenly establish trunk with the switch you manage. This is typical standard procedure when such switch port serves end users such as PC, printer, or servers.
In addition, by set specific switch ports as either trunk or access mode, you will have more control of how the switch should behave when a device connects to such switch port. By letting default setting in place (the "dynamic desirable"), there will be less control you have in terms of switch port behavior.