Your teeth develop to work in balance with each other. Different teeth serve different purposes, with incisors handling the gripping and tearing of food, while your molars take care of the chewing. They also share a proportionate degree of bite pressure, so that the load is more or less evenly shared. When some (or all) of your teeth have been replaced with dentures, the various components of your dentures will largely replicate this balance and distribution of bite pressure. However, this balance can sometimes become uneven, leading to a few potential problems. 

Your Denture Plate

The denture plate is the pink acrylic base that holds the prosthetic teeth. This plate mostly stays in position by itself, since it's intended to fit the contours of your mouth. There's an inevitable layer of saliva beneath the denture plate, and this creates suction, which also stabilises your dentures. Since upper dentures have a larger plate (with more surface area, and more suction), this upper dental arch is usually the most stable.

Leverage

Both upper and lower denture plates apply bite pressure by leveraging themselves against the underlying tissue and bone. This is what helps them to function as teeth. Dentures that don't fit as well as they should can create an imbalance with this leveraging, as the dentures are subtly shifting position when you apply bite pressure, which is when the balance of this pressure can become uneven.

Undue Force

At best, this uneven pressure will be mildly irritating. Your dentures will be slipping while you bite, and this can be annoying. At worst, this uneven pressure will apply undue force to your denture plate, and this can ultimately lead to hairline cracks forming in the acrylic base.

Hairline Cracks

Denture repairs are necessary for these hairline cracks. Such a crack can easily deepen and can result in the denture plate breaking into pieces. This can actually be potentially dangerous if it was to happen while your dentures are in your mouth. Hairline cracks are not always obvious, so it's beneficial to carefully inspect your dentures in adequate light. You can do this when you take them out to soak overnight.

Denture Relining

Hairline cracks are easy enough to repair—which isn't the case when the plate actually breaks. A complete break will often mean you'll need new dentures. Ensuring that your dentures fit well will help to prevent uneven pressure distribution, which can prevent hairline cracks. Remember that dentures should be periodically relined, which is when a thin layer of resin is added to the denture plate. This relining is the best way to ensure the best fit.

Your dentures should be able to evenly distribute bite pressure, and uneven distribution of this pressure can easily result in damage.