Spanning Tree on the interface that resides at end users I do not understand why we use portfast and in fact STP on the interface of a switch that is directly connected to end users. Indeed, a user can interfere with the STP election and perform DDoS (that is prevented by BPDU Guard). This is just not possible to disable STP on the interface that face the client ? Why portfast and STP are perform on interfaces that face end users ? I'm not really good with layer 2. Thanks for your help.

Portfast is used to decrease the time it takes for the port to move into forwarding mode. BPDU Guard will prevent DDoS or incorrect cabling. Both features are used to prevent loops in a network, while saving time on ports that connect to end user devices. Disabling STP will do exactly the opposite, open the possibility of having loops. A loop can easily bring down the whole network, either intentionally or by accident. If a switch is installed in a perfect world, where no one ever makes a mistake, then yes, you can disable STP on the switch.