Why do submarines use radar?

In WWII & Korea, submarines used Radar primarily for search operations - ships, surfaced submarines, and especially planes, which were a major threat. Long range detection of an incoming plane allowed the boat to dive to a safe depth and avoid being detected and attacked. They also used it for navigation close to shore.

Modern submarines today use Radar for surfaced navigational operations, typically when transiting from port to their dive point, or from their surface point to the port they're going to. This is primarily for surface ship detection, but in the case of entering a military port, it's for surfaced submarine detection as well. Modern submarines transmit few active signals even when entering port, and those are usually short-range civilian signals (VHF, etc.). Radar is also used in conjunction with GPS and other systems to gain a precise navigational fix when close to shore regardless of weather. No single system is ever relied on for anything onboard a submarine.

Submarines are not easy to see in the water, even surfaced, due to their low cross-section and color (black for most of the time, but their are camouflage paint patterns that can be applied as well in certain circumstances). Surfaced transit during the night require that submarines use a flashing yellow beacon atop one of the masts to denote to other ships that a surfaced submarine is in the area. Having done many small-boat transfers at night on a black submarine deck, I can honestly say it's impossible to see anything of the hull in the dark, and even barely with the light flashing, not to mention what it's like trying to work in pitch-black conditions when you can't see your feet and the deck.

As such, it is important that the crew know exactly what surface ships are where, and their range, so that proper action can be taken to avoid collision. When the boat nears the port it's heading for, Radar is used for establishing precise fixes to compare with GPS, charts, and other systems. Remember that most submarines are nuclear-powered - while the threat of collision when surfaced isn't that high, the problem of running aground is, especially if you're entering an unfamiliar port. Such an occurrence usually results in the relief of Commanding Officer.