If this has been discussed, my apologies.  My internet sucks worse than Joy Behar.

If I have a 20 inch .308 bolt gun, why do I need a DMR style AR?

DMR style would be 5.56.

Semis are for faster follow up shots. If that’s not a concern to you…

You don’t. Get both.

My 6.5CM Aero M5E1 is more accurate than my custom 6.5CM Bergara Premier.  Not by much, but it is.

Then again the Aero M5E1 has a Craddock Precision barrel headspaced to a specific load & bolt and weighs probably 16+ LB with scope, can, and bipod while the Bergara Premier is probably over 6” shorter OAL and weighs 9-10 LB with scope, bipod ,and silencer.

That’s the way to go about it, IMO… make one shorter and handier for intermediate work and walking about and then make the other pure precision.

I had a .308 battle rifle, and was going to convert it.  But I needed the money more than another .308 rifle.  I got a Remington 700 for my 50th bday and didn't think much about it after that.

Variety and diversity.

After I bought my first rifle I didn’t “need” any of the others that followed. But, like a good wood worker, I like to have tools for specific jobs.