Requiring a password for purchases will let you control who can buy what.
There are a lot of things to spend money on inside the Google Play store app on your Android. Some apps cost a few bucks, some have in-app purchases (some have too many) and others have subscriptions for monthly or yearly fees. Like everything else that can cost money, it's always a good idea to keep tabs on where and how the money is being spent — especially if you have kids using your Google account.
Thankfully, this is pretty simple to do. The very first time you use your Google account and the associated Google Wallet payment method, you'll get a confirmation window telling you that you need to enter your password to verify the transaction. You're also reminded that you'll need to re-enter your password for purchases after 30-minutes have gone by. This is the default setting, and while a 30-minute window is probably sufficient for most cases, your kids can rack up quite a bill in a half hour. Trust me.
To prevent this, you can change the setting so that a password (your Google account password) is required for every purchase, no matter how little time has passed since the last one. Fire up the Google Play Store app on your Android, and slide out the settings menu. In the User Controls section, you'll see an entry marked Require authentication for purchases. Tap on it. In the dialog that pops up, choose the setting that works best for you — always require a password, require a password every 30 minutes, or never require a password.
To edit this setting you'll need to supply your password, so as long as you've kept it a bit of a secret nobody can change any settings except you. It's a great way to keep credit card charges to a minimum if your kids are using your Google account!