It is the final coat in nail painting. It keeps your nail color from chipping away or turning pale and dull. Top coat is applied after you are done with colouring your nails. It hardens as it dries which is why it protects nail colour from scratches, chipping and peeling. It also strengthens nails and keeps the polish looking shiny and glossy especially when you are applying dark colours like black which tend to chip easily and look very tacky.
Excuse of most people for not applying top coat is that it takes longer to dry the nail polish but the truth is that lot of top coats will actually make your nail polish dry even faster. Top coats seal your polish off from the hazards of our daily life. We expect our nail polish to hold up to lot–showers, hand washing, exercise, cooking, rooting around in our bags looking for keys. A top coat is that extra coat armour. It’s not just your polish that it protects. The extra layer of strong top coat also lends stability to your nails themselves, protecting them from breaking so you can grow long, gorgeous nails.
There are also nail paint and Top Coat combo like Miracle Gel by Sally Hansen which is supposed be worn without a base coat underneath and they guarantee color upto 14 days without chipping away. There are also various other types of top coats available in market. Each one have different drying time and give different final look. If some of them gives very good look but takes longer to dry. You can think about using a UV or LED nail dryer.
Many top coats are UV light shielding and will keep your colors as bright as when you first applied them. Avoid products intended to be an all-in-one base and top coat. A base coat should be adhesive. Its job is to be the perfect foundation for things like nail polish to stick to. A top coat should be slick and tough and should slide off of anything that hits it. They have totally different purposes and trying to use one for both will only wind up with less than average quality layers of both.