Question

astronauts might visit Mars One Day. why would they need to take their own air​

Answer 1

Simple....

Cause There's No Evidence of an Atmosphere on Mars..

Answer 2

small plants in a comfortable pouch where the astronaut takes the plant's O2 and plants gets his CO2, or build some big scale factories where the factories generate green house gases and the gases take major hand to grow marse atm temparture and the plants will create their own ecosystem life cycle.