Why do some blockchains discard blocks created at the same time?

In a traditional linear-chain blockchain, when two miners produce blocks at the same moment, only one block survives — the other is 'orphaned,' meaning it is discarded and the mining work that created it contributes no security to the network. This happens because a linear chain requires all blocks to form a single unbroken sequence; parallel blocks cannot coexist, so the network must pick one and throw the other away. The winning block is chosen through a simple rule: whichever chain has the most accumulated proof-of-work — the 'longest chain' — is accepted as the true version of history. For a beginner, this matters because it means a meaningful portion of real mining effort is routinely wasted whenever two miners happen to find a block at the same time.