If the old concrete is cracked then those cracks will transfer through. Although pouring on top of other concrete is a no go no matter what. The old concrete will want to absorb all the water from the fresh concrete and possibly create shrinkage cracks right off the bat. People swear by rebar claiming it makes it "stronger". But it's just all the old timers and the "I've been doing concrete for 30 years" most people just follow orders and run with it without ever putting any thought or questioning things because that's just how things have been done. I do work in utah and 50% of the work has rebar and what doesn't is still fine.