Why we need start/stop bit for asynchronous transmission
If you didn't have a zero start bit that kicked-off the timing to the receiver, how would you know what to do when a serial byte comes along with a leading 1 digit in the data stream? What happens if the next bit is also 1 and the bit after that - what if all the bits are 1? Then you'd miss the entire byte because nothing would change (due to not having a start bit of 0).

Regular 8 high bits with leading zero start bit: -



Missing start bit: -