WHY DOES THIS HAPPEN?
Spindly plants with sparse foliage can have a few different causes, most commonly that your plant is being underfertilized and / or that it is not getting enough light. This, in turn, can lead to further health problems with your plants.
If your plant is kept somewhere that is too dark, it will stretch itself out to try and reach any available light source nearby, leaving it looking long and thin. Additionally, if your plant isn't getting enough light (or enough nutrients through fertilization) then it won't have sufficient energy to grow healthy foliage, which is why it can start to look leggy and sparse.
However, some plant types may actually become spindly regardless of their health. For instance, plants that die back in the winter can become spindly when they experience a growth spurt in springtime.
While spindliness isn't always necessarily a problem, it can affect the appearance of your plant and, if left unchecked, could eventually cause your plant to become top-heavy and at risk of toppling over. If this is the case, it's probably a good idea to repot your plant. Also, if your plant doesn't have enough leaves then this may affect its ability to photosynthesize properly.