Safety & flexibility
The biggest reason for a PPL holder to do the IMC is the extra safety it brings if you find yourself painted into a corner by circumstances beyond your control.
The rating also gives you the flexibility to fly more safely, for instance on days were the low level visibility is poor and all the traffic is flying around at 1500-2000 ft below a (just) broken cloud base the option of cruising at 5000 ft or so in the very clear VFR makes things very much safer and you know that should the broken cloud turn into overcast you can always get on the ground safely.
A few years back I had to break off an ILS approach at 600 ft ( still IMC ) a VFR pilot who had been caught out by rapidly deteriorating weather had in fear of his life turned base leg to final ahead of me, surely if the guy had done the IMC training he would at soon as VFR flight had become untenable climbed to the MSA and arranged to get onto the ILS in and orderly manor.
It was very fortunate for us both that he has the presents of mind to make a few radio calls because ( as I discovered on my next approach) the cloud base was about 270 ft with 5000m vis in RADZ, had I continued the first approach the chances are we would have collided just short of the runway.
It is all very well for those above to worry about being current and the cost of the rating renewal but the training never completely leaves you and when the VFR flight you are on is no longer a VFR flight you are far better to climb above the MSA, declare an emergency and get help from an ATC radar, calm down and then make an approach ( Preferably a PAR) than to continue scud running until you run out of luck, the administrative state of your IMC rating is not a factor under these conditions..............it is staying alive that counts !
The fact that this essential piece of pilot skill is about to be watered down because of pressure from EU states with a far worse safety record than the UK just illustrates the fact that EASA is not a safety regulator it is just another part of the EU cushy well paid job club.