What is rooting in android?
Rooting is the process of getting access to the"root" user. when you "root" your phone you are unlocking the administrative privileges of the phone which by default are locked by the manufacturer. It's similar to 'Jail breaking' in the iPhone.Rooting your device allows you to do things like

• Overclock or underclock your CPU.

• Remove those preloaded/Bloatware apps that your phone can't delete.

• Install certain apps that require root access.

• Making Nandroid Backup

• Load custom ROMs. and many more

The manufacturers don't want you to have access to these options for a number of reasons like

• They want to control and ensure a specific user experience.

• Certain carriers require that certain functions are locked out because they want you to pay for them or because they are incompatible with their network and many more.

The only risk is bricking your phone.


Benefits of Rooting

•  Unlock Hidden Features and Install "Incompatible" apps 

Sometimes even Android isn't open enough to give you some of the features you want. Either an app is blocked by carriers, hacks into Android's system files, or otherwise isn't available. Luckily, rooting can help with that: you can install carrier-blocked apps,get features from the latest version of Android,make incompatible apps compatible,power up your hardware,get features like Beats Audio from other phones, or emulate exclusive features.Whatever you want, rooting gives you the power to do a lot more.

• Automate Everything You've probably heard of Tasker, the awesome app that automates just about anything on your phone.

You don't need to root your phone to use it, but if you're rooted, it can do a whole lot more. Certain tasks, like toggling 3G, GPS, changing CPU speed, turning the screen on, and others require root access. So, if you want to get the full benefit of an app like Tasker, you'll definitely want to root your phone.

•  Boost Your Phone's Speed and Battery Life

App like SetCPU you can overclock your phone for better performance, or underclock it for better battery life. You can also use an app like Greenify to automatically hibernate apps you aren't using perfect for those apps that always want to run in the background when you're not looking.

Also know the useful root apps Must have root apps for android

• Back Up Your Phone for like a pro

When you move to a new Android device or restore your device to stock for any reason you can make a lot easier by backing up your apps and settings. That way, you can get your entire setup back easily. If you aren't rooted, you can back up a few things like apps and data, but you won't necessarily be able to backup system apps and their data.

• Remove Preinstalled Bloatware

It can also uninstall that annoying, battery-draining, space-wasting bloatware that comes preinstalled on so many phones, you can remove the apps and make phone faster and also save more space.

• Flash a Custom ROM

One of the best benefits of rooting. You can try variety of custom ROMs A custom ROM is basically a custom version of Android, and it truly changes how you use your phone. Some merely bring a stock version of Android to non-stock phones, or later versions of Android to phones that don't have it yet.Some add a few handy features,some add lots of really unique features, and some change your operating system from head to toe. No matter what phone you have—even if it's a Nexus—we highly recommend checking out the custom ROMs out there. You won't be disappointed.Note: As some of you have noted, you don't actually need root access to flash a custom ROM—though you will need to unlock your bootloader (a process that sometimes comes bundled with root access). Still, it requires freeing your device from manufacturer lockdowns, so we've kept it in the list despite this technicality!How to Flash a ROM to Your Android Phone Android is great, but sometimes, the version you get with your phone—whether its vanilla Android…Read more1. Truly Own Your Device In the end, all of this boils down to one thing: you own your device, and you should be able to do with it as you please. Certain manufacturers and carriers try to keep that from happening, but with root access, you truly own your device and open yourself up to all the possibilities other parties try to block. Sure,there's some risk involved, and we don't usually recommend rooting other people's phones, but in the end

• Rooting allows you to install “unofficial” and advanced versions of the Android OS.

Still using old version of android and not getting any update, wanna try new versions of of android then rooting is your answer it allows to do install new android version ROMs seamlessly.  
Rooting lets you fully personalize and customize your phone’s interface with cool themes, wallpapers, widgets and more.

 Know more about Best Custom ROMs

Interested in rooting see How to root Android device 

Disadvantages of Rooting

• Rooting might void your phone’s warranty.

• Rooting can cause your phone to act abnormally, especially if not done properly.

• Rooting renders your phone unqualified to receive OS updates through OTA. So you will be on your own to look out for valid sources. The good news is that your phone is probably running the latest OS after you have rooted it.

• Rooting may make your phone perform faster but there are downsides to it specifically running too hot to the point of overheating your device.