Syncing Android data with Google account helps you keep your phone, Google + contacts and other files synced with your Google account, the one you're using the Google Play Store and with all the other Google services.
Why Syncing Android is Necessary?
During syncing, much of data on your Android phone or tablet is backed up by Google automatically. It is necessary because you may lose your data files, text messages and contacts while rooting or updating firmware. If mishap happened, you can easily restore them back from Google account.
What Google Syncs/Backups Automatically
Google backs up most types of data you care about associates it with the appropriate Google service, where you can access it on the web.
Contacts, Email, and Calendars: Your Android contacts are synced with your Google contacts online, your email is safely stored in your Gmail account, and calendar events are synced with Google Calendar.
Chrome Browser Data: If you use the Chrome browser, your bookmarks are synced with your Chrome sync account. In newer versions of Android on some phones, the Android Stock browser syncs with your Chrome bookmarks as well.
Google Talk Chat Logs: Google Talk chat logs are stored in your Gmail account, assuming you haven't disabled chat logging in Gmail.
Some System Settings: Android also syncs some other system settings – for example, passphrases for Wi-Fi networks.
Apps and Other Purchased Content: Apps and other content you purchased from Google Play are tied to your Google account. When you set up a new Android device or enter your account after resetting your Android device to factory reset, Android will automatically download and install the apps you previously had installed.
What Google Doesn't Syncs/Backups
Now that we're covered what Google does automatically back up, let's take a look at what they don't:
SMS Messages: Android doesn't back up your text messages by default. If having a copy of you text messages is important to you, you should utilize a free app named SMS Backup+.
Google Authenticator Data: For security concern, Google doesn't sync your Google Authenticator codes online. If you lost perform two-step authentication after wiping your device, you can authenticate via SMS or a printed authentication code, and then set up a new device.
Game Progress: If you need to move your games to a new phone, you will notice that your game progress was erased.
Logins: You will have to enter password and log into the apps you use each time you get a new device or restore your device to factory defaults.
App Settings: Many other app settings aren't backed up by default.
How to Sync Contacts on Android with Gmail?
Actually, if you sign into a Google account and enable Auto-sync, all the data that can be backed up are synced with Google. Here we take syncing your contacts for example.
Make sure that you have Gmail installed on your device.
Open the App Drawer and go to Settings, then go to "Accounts and Sync". (You can also check "Auto-sync" directly and finish the steps).
Enable the Accounts and syncing service.
Select your Gmail account from the e-mail accounts setup.
Make sure that you've enabled "Sync Contacts" option.
Press "Sync now" button and wait until all the phone contacts have been successfully synchronized with your Gmail account.
Now open Gmail in your computer's web browser and click the Gmail text link that is listed on the upper left side of the Gmail profile.
Select Contacts and then you should see a page where all the contacts from your Android smartphone have been stored.
If you lost your contacts and didn't sync them with Gmail, don't worry, you can get back all of them by using Android Data Recovery.