Regular rotation helps to even out the wear of all 4 tyres.
If you don't do it often (like me) or not at all, you will always end up with 2 badly worn and 2 less worn tyres. So when it's time to change the 2 badly worn tyres, you will have to:
1) change the 2 bad tyres, having 2 new and 2 old tyres, creating less ideal grip imo, and the cycle continues.
Or
2) change all 4, like I do, wasting money. In my case, my car odometer reads 45k km, and I'll be changing my 3rd set of tyres since I got the car brand new. 2 tyres have reached the wear indicator whereas the other 2 still have 1-2mm above the wear indicator.

Another issue is when your tyre model is discontinued and you will have to use 2 types of tyres. Is it good? Certainly not ideal imo.

Imo best to rotate using the pattern (FR to RR, FL to RL, RR to FL, RL to FR) in order to fix the tyre at all 4 positions by the 4th tyre rotation.

I agree with him. It's too late for you to rotate now, as it will mean the back ones (after rotation) will be in worse condition than front tyres. Easier to lose control of the car.