Whether because you suddenly realise you've left a dodgy tab open on an iPad you've just handed to a colleague or because it's just flat-out easier to go through and close a bunch of tabs on your Mac rather than on an iOS device, you should know that you can close tabs open on any device signed into your Apple ID from Safari since Yosemite.
Click the icon that looks like two overlapping squares in Safari (or choose Show All Tabs from the View menu) and you'll see all your open tabs on all your devices.
Click the icon that looks like two overlapping squares in Safari (or choose Show All Tabs from the View menu) and you'll see all your open tabs on all your devices.
Hover over each and you'll see a close button you can click. (This also works from iOS to Mac; swipe right to left on a cloud tab in its tab view and tap Delete; that tab will then be closed on the Mac.)