Submarines made out of titanium, instead of steel, can of faster or dive deeper. This can give them a tactical, even strategic, advantage. And it makes them able to carry out special missions. But only the USSR, and now Russia, can build them.
This Covert Shores submarine chat covers all the classes of submarine built out of titanium. And why.
Article on ALFA Class high-performance submarine: http://www.hisutton.com/Alfa_Class_Submarine.html
Article on SIERRA class submarine: http://www.hisutton.com/Russian%20SIERRA%20Class%20Titanium-hulled%20attack%20sub.html
Article on unique Losharik deep-diving special submarine: http://www.hisutton.com/Spy%20Sub%20-%20Project%2010831%20Losharik.html
Article on MIKE Class deep-diving attack submarine: http://www.hisutton.com/K-278_Komsomolets.html
Article on Piranha special forces submarine: http://www.hisutton.com/Piranha%20Special%20Forces%20submarine.html