What will happen if I don't use RS232 driver for UART?
The UART is the brain. 

Your brain takes raw information received from sensory information from somewhere and converts it into something you understand and can react to. When you read this text you are decoding the alphabetic cypher information that are words and extracting meaning from it. A UART does a similar thing with the ones and zeros on a digital line.

We use many communication methods to transmit our human information over a distance. Things like this forum, the internet, or a telephone system. 

Each communication method has their own standards they employ to convert the information from the sender into another form that can be sent over some distance and then converted back, hopefully error free, at the receiver end.

The RS232 standard is one of those standards. It defines how a signal is to be generated for transmission down a pair of wires and how that signal should be received at the other end. However, RS232 itself has no idea what the signal itself means.

Historically, RS323 and UART sort of got lumped into the same meaning since a Serial Port (which by the way has it's own standard over and above the rest) was predominantly a UART + RS232 for many decades. However, it is important to understand that the RS232 part is merely a signal specification. You can in fact send any digital signal via RS232 signal levels.

The UART outputs and receives simple digital signals. For short distances, inches to maybe a foot or so, you do not need to convert to RS232 or any other communication standard. 

Like two people chatting in the same room, the range of your voice is usually fine. If you want to talk to your spouse at work on the other side of the city, you need a phone, or a computer.