Going submerged avoids detection by sight - which in turn was partly neutralized by night and bad weather. Uboats were very low on water and hard to spot in bad weather.

Going on the surface also makes detection by ASDIC (passive and active sonars) impossible.

One more thing, submarines of that type didn't have very great max submerged speed - something like 7 knots compared to 17 on the surface, and not too much underwater endurance either.

Dan