The Inside AdWords blog tells us
How Shared Budgets Work
Say you’re an outdoor furniture seller with a single line of products. You’re currently running three campaigns:
A desktop search campaign
A mobile search campaign
A remarketing campaign to reach people who have visited your site but didn’t convert
Your overall marketing plan allows you to spend $100 per day across your three campaigns. Without shared budgets, you’d next have to decide how to allocate the $100 daily AdWords budget across each of your three campaigns. Say you set a $60 daily budget for your desktop campaign, a $20 daily budget for your mobile campaign, and $20 to your remarketing campaign.
On most days, each campaign hits its daily budget and you’re satisfied with the ROI of each campaign. But on some days, your desktop search campaign sees fewer impressions and clicks than other days. So you only spend $90. On these days, your overall campaign results could be stronger if you were able to put an additional $10 into your mobile search campaign or remarketing campaign.
Translation: We, Google, are making sure that you can spend every dollar you intend to spend with us. We are simply facilitating the safe transfer of all of your online budget into our bank accounts ……. for your benefit of course!
While that is a bit sarcastic it is true but it also makes sense if you can get better performance from a campaign.
In reality, if anyone is surprised at a move from Google that helps advertisers spend more money with them then they are pretty much in the dark as it relates to the online world according to Google.