You can only use blood of the same type; with the exception of O- blood which is universal and rare.
https://www.redcrossblood.org/donate-blood/blood-types.html
Only about 7 per cent of Americans have that blood type, so donations are important because it’s necessary for emergency situations where there is no time for blood typing.
O+ is actually the most needed.
I used to donate regularly. It’s not a big deal to donate blood. Once in a while you feel light headed but they give you cookies and juice and 20 minutes later you’re fine again. Sadly due to medical reasons and medication I take for my heart I am no longer allowed to give per protocol. I would if I could.
Young people in good health should donate. It’s a simple thing that is beneficial to the common good of the country.