Here's a real-world example where atomic updates aren't an option - an order status transition that reads the current status from one table, validates the transition, and inserts into another:

await db().transaction(async (tx) => { await hooks?.onTxBegin?.();

  const [order] = await tx.select().from(orders)
    .where(eq(orders.id, input.id))
    .for("update");

  const [status] = await tx.select().from(orderStatuses)
    .where(eq(orderStatuses.orderId, input.id))
    .orderBy(desc(orderStatuses.createdAt))
    .limit(1);

  if (input.status === status.code)
    throw new Error("Status already set");

  await tx.insert(orderStatuses).values({ ... });

});

You need the transaction + SELECT FOR UPDATE because the validation depends on current state, and two concurrent requests could both pass the duplicate check. The hooks parameter is the barrier injection point from the article - that's how you test that the lock actually prevents the race.