What is the purpose of OP_DROP
The TIMEOUTOP, which is one of OP_CHECKSEQUENCEVERIFY or OP_CHECKLOCKTIMEVERIFY, will either:


Terminate the script with an error if the TIMEOUT conditions are not met
Act as a NOP and leave the stack unchanged if the TIMEOUT conditions are met


2 occurs if the timeout has been met, and leaves the top of the stack with the timeout delay value. OP_DROP removes this value from the top of the stack, allowing the regular p2pkh script to continue execution by consuming the rest of the stack.