Google multiple sign-in is a handy feature if you have more than one Google account and you want to switch between them without logging in and out every time. (This log in and out action is still needed for a technical reason which I will point out at the end.) I use it often to switch between my accounts when checking emails (in this way, you can easily have multiple Gmail storage). Note that you need to enable multiple sign-in for your accounts at the account setting page before you can use this feature.
|The switch account button is highlighted on the right.|
|On the list of signed in accounts, the one that you first signed into is marked with (default).|
Although this feature is convenient, it is not supported uniformly well across all Google services. In any case, all sites will recognize your account as the default account which is the one that was first signed in and some of them will let you switch between signed in accounts. The implementation on the popular services such as Gmail, Google docs, and Google Calendar are seamless but not so for some others like Google Wallet (a.k.a. Google checkout). In fact, it can be really confusing sometimes.
If you want to change the default signed in account, there is no click-a-button method to accomplish that at the moment (which should be implemented soon by Google I hope) and so you have to log out (which logs you out of all signed in accounts) and log into the account you wish to make default.