Problem understanding shiftRight function.

Hello, while writing with JavaScript, I encountered the following shiftRight() function. The final section clears the last character with a dash. This is because it is overwritten. I am not understanding why this is needed, I tried to research but my efforts were unpromising. Thanks!

// === Shift all characters one space to the right (after insertion) ===
function shiftRight(row, col) {
  // Flattened index of the insertion point
  // Shift everything from this point to the right by 1
  for (let i = ROWS * COLS - 2; i >= row * COLS + col; i--) {
    const from = i;       // Current character index
    const to = i + 1;     // Destination index (1 cell to the right)

    // Convert 1D index to 2D row and column
    const fr = Math.floor(from / COLS); // From row
    const fc = from % COLS;             // From col
    const tr = Math.floor(to / COLS);   // To row
    const tc = to % COLS;               // To col

    // Move character to the right
    grid[tr][tc] = grid[fr][fc];
  }

  // Clear the last cell in the grid
  const lastIndex = ROWS * COLS - 1;         // Final 1D index of the grid
  const lastR = Math.floor(lastIndex / COLS); // Last row
  const lastC = lastIndex % COLS;             // Last col
  grid[lastR][lastC] = DASH;
}