Luggage, to me, protects the contents and makes them portable. Ideally a bag is durable, easy to maneuver, and the right size — if it’s a carry on bag then it should fit in airplane overhead bins (ideally even large regional jet bins).
Your bag should be sturdy enough to withstand the beating it gets from continued use and the abuse of being overstuffed. If checked it should be able to withstand the abuse it gets from baggage handlers, weather outside on the tarmac, and the TSA.
How do you feel about bags: cheapest possible, pay more for something agile, or invest in designer luggage?