Fasting gives your body a break, some people liken it to pressing the reset button. When you stop consuming food, your body starts to work on the debris that has inevitably started clogging our bodies because of our diet and environment.
The main benefit is that you will probably feel amazing afterwards. Depending on your situation you could see improvement in some of your health issues as well.
A demerit I would say is that you usually have to go through a phase of feeling pretty bad before you feel good. This is attributed to the toxins being freed from wherever your body has stashed them and moving through your system as it finally eliminates them. Generally people feel dizzy, nauseous, lethargic, have bad breath etc during the initial stages of cleansing.
If you are considering it for the first time, make sure you are ready and do your research first. Most experts will recommend you to do a cleanse first before fasting. A cleanse will involve many things including an alkaline diet free of processed (processed includes cooked) foods. The cleanse will move you gradually from eating solid to liquid food, and finally to a state where it is safer for your body to attempt a full on fast.