because you need to do it if you want to show contourlines. Now compiling the worldwide contourlines each time again - would burn SSD as well - besides taking longer than just compiling the maps. So everyone who offers maps for many countries as download puts the contourlines separate. If you want to offer the choice of showing contourlines or not - mkmgap will write the gmap contourlines once not needed, and once the maps not needed. If you don't want to offer that option - it's only the contourlines being written without need. Contourlines for all geofabrik extracts 20m equidistance are about 130GB, 10m would be 260GB.
If you offer 10m and 20m it will be even more not needed writes.
The only solution is to go for a Ramdisk instead - However you likely will need 128GB of RAM to do that - because for Asia or Europe you need a 64GB Ramdisk. Same for North America extract (but few people offer that and just have Canada and the US divided into the 4-5 zones). For other maps you will get by with a 15-20GB Ramdisk (which I have resorted to now for all but the windows installers because I don't want to let the server wait for ages for the single thread NSIS wrapping up the data and instead start the next country in parallel). And yes going RAMDISK already using my patch and symlinking those files so mkgmap cannot overwrite them (would still be faster if mkgmap didn't try in first place I think). If you want to write out the contourlines as well besides the additional time - for Asia continent you may then go for a 90GB Ramdisk minimum if offering windows and gmap installers. In this case gmapsupp.img donwnloads aren't possible anyhow due to the huge data amounts. For smaller countries I have them too....
I coded around this now by using symlinks - but yeah that will be quite a lot of work and is prone to break - while I guess it's only 10 lines or so to add to mkgmap code to have a mode that does not write them out if they are present - or if you tell on commandline they are present already. For .img files they aren't overwritten either...