What is the use of submarines today?

Depends on what class of Submarine you are talking about.

Balistic Submarines are a nuclear deturent that assures retaliation in the event of a country using nuclear weapons against one another.

Attack Submarines have grown a lot since WWII. They are now capable of conducting Anti-Surface Warfare, Anti-Submarine Warfare, Land Attack Strikes, Harbor Mining, Speacial Forces Insurcian & Extraction, & Intelligance Gathering.

Scientific Research Submarines are used for Biological Research, Search and Recovery of Shipwrecks.