Before I moan, FWIW I looooove riding on and working on shimano hubs, but I still think they didn’t think their 15 and 20mm ones through properly.
Leku you are right, this tool it is the only effective way of holding the axle still whilst you do the cone and lockring up on the other side. You would do this on a shimano qr hub using a vice or third cone spanner on the opposite side to the one you are working on, to hold the axle still as you tighten the outer nut against the cone. Of course on shimano 15 and 20mm hubs there are no spanner flats to be able to do this, and so the £40 tool is there to hold the axle still in a vice or the jaws of an adjustable spanner.
You can do it by ‘trial and error’ ie start with the cone a bit too tight and expect it to loosen itself a bit as you tighten the lockring a bit. That is a bit of a faff requiring several attempts to get it just right, and doesn’t always work or stay put very well (in my experience of no problems whatsoever with lovely shimano qr hubs and plenty of grief with the slx and xt 20mm hubs I had.)
You can also do what I resorted to doing and write off your warranty by sawing a 2mm wide by 3mm deep slot into the disc side of the axle. Then put a spare cone spanner (or anything else made of <2mm hard steel) in a vice with a bit sticking out of the top of the jaws of the vice, and put the slot you have cut in the axle on the exposed bit of spanner. This way you can tighten the cone and locknut onto each other without the axle rotating.
What a shame that to service these hubs properly yourself you have to spend a total of £51 on unusual tools, (the slx, zee and xt hubs all cost less than that! What other hubs are there that require you to spend more on the tools than you did on the hub?!) or £11 on just the massive cone spanner and saw a slot in the hub. 😕