Manned spacecraft carry helium tanks. Why?

below are some examples, but most of those involve the rockets used to get into space.

A rocket in space uses Helium in more than one way. In its three stages of launch, the first stage uses Helium to purge fuel gases comprising of Hydrogen mixed with Oxygen in the fuel vessel. It is pumped into a rocket fuel tank to create pressure, which is required for firing the rocket. Helium slowly increases pressure and can be used to control the flammability of Hydrogen. A Saturn booster, like the type used on the Apollo Lunar missions, required about 13 million cubic feet of Helium for firing and more for checkouts.

Helium is also used as a coolant in Hydrogen fuelled rockets. During operation of a satellite in space, cryogenic properties of Helium help in avoiding overheating of its components. Being an inert gas it does not react with the other chemicals present and can be safely used. Space borne Helium cryostats are designed to store super fluid Helium. They are compact and made light weight using aluminium alloys casings.

Helium cryostats are specially designed for space applications in many other optical equipment in the flight on board. The casings have multi-layer volume insulation that allows Helium to stay at low temperatures for several years in Space. High technology devices and porous plugs are designed to control the flow and avoid any probability of leakage. It is expected that not even one milligram of Helium would leak from the cryostat over the hundred years.

Astronomical bodies moving towards satellites are detected using infrared sensors. Normal cosmic radiation occurs at 3K and cooling of infrared detectors to 2K using liquid Helium is another useful application. Spacecraft also use Helium in specially designed magnetometers for space. The Helium Vector Magnetometer can sense the altitude of a satellite; help in detecting other satellites and also help in measuring magnetic fields of Earth, Moon, Sun and Planets. Helium ionized gas magnetometers have been used to detect radiation belts around the Earth.

To "Cut" the Oxygen percent to 20% or so for breathing, like on EARTH. On Earth the air is 20% Oxygen--it is mostly Nitrogen-----But in space, with less pressure, Nitrogen can form bubbles in the blood on certain conditions. This mixture is called "Heliox". Deep sea divers use Heliox also to avoid Nitrogen narcosis and to help avoid the bends as they rise, to where the water pressure is lower, and the nitrogen in their systems comes out of being dissolved in the blood.------

Low spaceship air pressures may make dissolved nitrogen have the same effect---so Helium is used instead of Nitrogen to make the atmosphere 20% oxygen.

Actually liquid nitrogen, used to pressurize fuel tanks.

Some very few applications where supercooling is needed liquid helium, mostly unmanned probes.

No, they don't. They have oxygen a d hydrazine tanks. There's no oxygen in space.

To pressurize the fuel tanks.