Why I need to add delay after HAL_UART_Transmit_DMA and before memset at IRQ call back function

void vcom_IRQHandler(void) // external interrupt for USART2_IRQHandler
{
	HAL_UART_IRQHandler(&UartHandle);		
	USER_UART_IRQHandler(&UartHandle); 
}
 
void USER_UART_IRQHandler(UART_HandleTypeDef *huart)
{
 if(USART2 == UartHandle.Instance) //Determine whether it is serial port 
 {
 if(RESET != __HAL_UART_GET_FLAG(&UartHandle, UART_FLAG_IDLE)) 
 {
 __HAL_UART_CLEAR_IDLEFLAG(&UartHandle); 
 USAR_UART_IDLECallback(huart); 
 }
 }
}
 
void USAR_UART_IDLECallback(UART_HandleTypeDef *huart)
{
	
 HAL_UART_DMAStop(&UartHandle); 
 
 
 uint8_t data_length = BUFFER_SIZE - __HAL_DMA_GET_COUNTER(&hdma_usart2_rx); 
 
 
 HAL_UART_Transmit_DMA(&UartHandle,(uint8_t *)&aRx2Buffer, data_length); 	 
 DelayMs(100); // why need this delay
 
 memset(aRx2Buffer,0,data_length); 
 data_length = 0;
 
 
 HAL_UART_Receive_DMA(&UartHandle, (uint8_t*)aRx2Buffer, 25); 
}

Scenario : After I call HAL_UART_Transmit_DMA, if I didn't put a delay at there, I monitor at serial terminal like docklight/putty, I won't see a full data.

And then I use JTAG to debug, step one by one, found out that I can receive actual data. (I think because of stepping is not run time anymore, and it indirectly perform an delay(s) at there)

So I add delay after that function, I can solve the problem. But I want to know the reason and also the drawback if I add the delay. To be honest I think a delay like that will halt the CPU for so long.