Correct. But engine flush is not only just meant for gunk, sludge or deposits; sometimes, old oil and the deposited acids / harmful products made in that oil with the passage of time; may react with the new oil, especially when the oil brand, grade and viscosity are going to be different from the current one.
Last time, when I changed from Eneos Sustina 5w-30 to Caltex Chevron 5w-40, I was a bit worried that old oil and its very minute harmful deposits (which even the oil filter might have missed) may react with the new one.
The guy himself advised me for complete engine flush, and when I asked about it's harmful effects on engine, he said:
"Bhai Sb. hamara rooz ka ka'am hai, aapki gari aur engine, dono naye hain. Kuj nai ho ga"
And he was right. I went for complete engine flush before switching to new oil grade / viscosity.
And Next time when I will be switching to 0w-20 or 5w-20, I will again go for complete engine flush I.A. as to remove even the tiniest fragments of old used Caltex Chevron 5w-40, most importantly, brand, grade / viscosity will be changing.
[Just for reference; on the front of STP® Engine Flush (as per http://www.stp.eu/en/products/oil-additives/engine-flush ), it has been written "Use with every oil change", so I think it is safe, especially for new engines or at least, when one is switching between oil brand / grade / viscosity]