Why does RS-232 need a stop bit?
The thing to remember is that RS232 is an asynchronous protocol. There is no clock signal associated with it.



Figure 1. Receiver sampling points. Source: Sangoma.

The start bit is used to trigger the read cycle in the receiver. The receiver synchronises itself on the start bit and then waits 1.5 cycles to start sampling bits. Thereafter the bits are sampled at the baud rate. This initial delay means that even with a 5% clock error the receiver should still be within the bit timing for the last bit.

Since the start bit - shown low in Figure 1. - is identified by a falling edge then it must be preceded by a high and this is what the stop bit ensures. The alternative would be two start bits and no stop bits but it wouldn't change the total message length.

The linked article has some other points worth noting.