Let us look at the history of game updates:
Update 0.5.4 (23 March 2016) - addition of Russian Cruisers. Radar Consumable is introduced for the first time, and currently only on Russian Tier VIII to Tier X cruisers
Update 0.5.4.1 (30 March 2016) - American Cruisers from Tier VIII to Tier X received Radar consumable
Update 0.5.5 (4 May 2016) - the "Great IJN DD Torpedo Nerf"
Update 0.6.0 (18 January 2017) - Captain skill rework. New skills such as IFHE and RPF added
So what's the thing before March 2016?
DDs had stealth-firing capabilities, IJN DDs farmed free damage for 0 risk from max range, and there is no smoke-firing penalty for all ships, which allows DDs (especially US DDs) to virtually hide its entire team in a flank in its smokescreen and deal significant damage to enemies without any risk. There was no safe counter to smoke, as cruisers charging into smoke with hydro more often than not gets killed off before it can detect the DD. Despite the lower average player skill at the time (since the game wasn't a year old after Open Beta), only less-skilled DD players allowed themselves to get "ambushed behind islands" by cruisers. Good DD players abused the aforementioned stealth-firing, stealth-torp farming and smoke-firing so much that Radar is added to alleviate this problem.