The spark plugs on some engines are sunk down into deep recesses that are so close-fitting that getting any kind of conventional wrench or socket on the spark plug's hex is nearly impossible – a real problem if you don't have the original plug wrench form the motorcycle's toolkit (if the motorcycle even had a toolkit.)
Similar spark plug locations on other engines may allow clearance for a traditional socket, but there's an obstacle in the way (camshaft/valve cover, frame member, close-fitting gas tank, etc...) that prevents the socket from slipping over the spark plug. It's therefore important that you buy a simple but clever solution. Buy a dedicated spark plug wrench with a removable hex-socket, with a center opening large enough to fit over the spark plug's insulator and walls thin enough to let the tool slip down onto the spark plug's hex. You then connect the hex-head to the wrench to turn the spark plug. The way the wrench connects to the hex-socket is for every wrench brand different, but the basic design is all the same, the hex-socket that holds the plug is easily removed from the socket just by pushing to one side or another.
Like most dedicated motorcycle/automotive tools, specialized spark plug wrenches with disconnectable hex-socket aren't cheap, but after you quickly and easily remove those difficult spark-plugs a few times, you'll consider that it was a worthwhile investment.