There are several problems with standard earphones. First is that because they are one size fits all they often don't fit your ears. This leads them to fall out easily, particularly if you are exercising when you are moving around, sweaty and the lead on your earphones is being repeatedly pulled. This is irritating, distracting and can result in your earphones becoming broken.
The next problem is that the sound from many lower end earphones is of poor quality and often sounds tinny. This is often due to the fact that the earphones do not create a good seal with your ear. Many earphones are round in cross-section, where the opening in your ear is oval and everybody's is a slightly different shape. This leads to discomfort and much of the higher energy sound leaking out around the earphone, rather than travelling down your ear-canal and vibrating your eardrum which turns it into the sound you hear. This leads to loss of much of the bass and midrange, leading to the classic tinny sound of cheap earphones. The third problem is ear infections.
Frequent and prolonged use of earphones can lead to otitis externa, also known as swimmers' ear. It is called this because it is common among swimmers whose ears become waterlogged; this is similar to having your earphones for prolonged periods of time because they become damp, particularly if exercising. Bacteria and fungus infect the ears; because the earphone is blocking the canal it is unable to dry out properly, leading to infection, inflammation, redness, soreness, itching, flaking and muffled hearing due to the buildup of wax and debris in the ear canal.
Moulded earphones overcome these problems in several ways. Because they are individually shaped to your ears they create a tight seal, hugging its contours tightly. This leads to reduced sound leakage, improving the bass and mid-range sounds. This also reduces the irritation of other passengers on the tube or bus as less of the sound escapes from your ears and more of it is directed towards your eardrum. This also allows you to play your music at a lower volume, prolonging the precious battery life of your smart phone or MP3 player.
The fitted shape of the earpiece is also less likely to fall out as it fits like a key into a lock in your ear, also making it far more comfortable. Some molded earpieces are antibacterial, reducing the chance of ear infections. Having your earpieces made by an audiologist can be an expensive and time-consuming process; you might not even get the results you would expect for the money that you pay. Kits are also available which allow you to mold your own at home for a fraction of the price.