New washing machine. Why we have to perform a cleaning cycle?
This is the same reason that any dishes or glassware that you buy says to wash it before you use it.  They really try to keep everything clean at the factory and chances are your glasses (or washer tub) are spotless, but they can't guarantee it.

To be absolutely sure your new white sheets don't end up with blobs of gear grease all over them because of a glob that wasn't noticed, they want you to to a hot wash with no clothes.  This cleans the tub and if there's a mechanical problem or leak, you find out with just water rather than 40lbs of wet clothes.

I would do the hot cycle as recommended just to be totally sure.  If you're just doing a load of old towels, maybe you can skip it since you did do a warm wash already.  Heat will help loosen oils and grease which is probably why they specify the 90C wash.