UART frame has 1 start bit, 8 bit payload data, 1 or 2 stop bits. Frame structure and baud rate is agreed by both transmitter and receiver for successful data transmission. Start bit is used to synchronize and tell receiver that frame is started. Why there is a need of stop bit per frame, since all data frame to be transmitted will always be 8 bits (frame will always end with 8 bits)?
(sorry: I am in teacher mode)
What will the line signal look like (without a stop bit) when you transmit a continuous stream of 0x00 bytes? which problem will this cause?
What is used for synchronization is not the start bit itself, but the falling edge between the previous stop bit and the start bit.
Without both stop and start bits, there might not be such an edge.
It's a holdover from the days of electromechanical teletypewriters, when the time was used to release a clutch in the electromechanical transmitter mechanism. It was carried forward into fully electronic UARTs, perhaps because the original electronic designers didn't understand electromechanical teletypewriters.
Here's a discussion of how the old teletypewriters worked: http://mysite.du.edu/~jcalvert/tel/teletype.htm