When the Ethernet concept was designed Redundancy was not considered, So Ethernet does not have a built in way to deal with Layer 2 loops
A Layer 2 loop can result in:
MAC address table instability
Link Saturation
High CPU Utilization (switches & end-devices)
Resulting in the network becoming unusable
STP was developed specifically as a loop prevention mechanism for Layer 2 Ethernet
Why do Race Cars Promote STP when they like to Drive in Loops?