If you are traveling on multiple airlines it is a good idea to check the baggage rules. Actually, you should do this anytime you travel. The rules are sometimes different from airline to airline. let's say you are traveling from the U.S. to Africa via a city in Europe. The U.S. carrier may have a different weight limit on your carry on bag from the carrier that will take you from the European city to Africa.
Thus it will be necessary to check your hand luggage. If you have already exceeded your checked baggage allowance, then you will be charged for an additional piece of luggage.
If you are not connecting, you should still check your hand baggage size and weight restriction with your airline. As if it is too big or too heavy, you will have to check it.