For a whole bunch of reasons, "weight" and "mass" can be treated as interchangeable when you do everything at 1g. That includes doing your shopping, or calculating an aeroplane's weight/mass and balance from a weighing on the ground. This is how the incorrect market metric unit of kilogrammes-force comes into such widespread use, and in imperial units so many people get confused between the pound-mass (lb) and pound-weight (lbf).
In flight however, we are regularly NOT at 1g, thus it's important for e.g. doing aircraft stability or performance calculations, we differentiate between the two. It's also where SI becomes suddenly massively superior to Imperial, since SI only has one unit for weight/force, the Newton, and one for mass, the kilogramme, whilst Imperial has two mass units (the slug and the pound) and two force units (the pound and the poundal).
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