Why are chair conformations more stable?
Because chair conformers minimize ring strain, and trans-annular interaction......
And the only way you can appreciate this is to make a model of cyclohexane, and substituted cyclohexanes, and have a play with them. For cyclohexane, the chair conformer gives 3 axial substituents, and 3 equatorial substituents, per face of the ring. Such models are always allowed in formal examinations. Of course, you have to be able to represent the model on the page clearly and straightforwardly. This is not a trivial exercise, and takes practice.