Multiple Listing Services are databases of homes for sale put together by real estate brokers. The primary purpose of an MLS is to make buying and selling a home as quick and easy as possible.
The listings in an MLS contain both general and detailed information about homes, as well as information about the broker. As long as you are either buying or selling your home through a broker, you will be able to take full advantage of an MLS.
However, you will not be able to have full access to an MLS if you are not a broker. Therefore, if you are looking to buy a house without a broker, you will have to do it the old fashioned way: looking at local listings online and in the paper. This gets very difficult to do when you begin expanding your search outside of your town and eventually out of state.
Selling a home without a broker, though possible and frequently done, is much harder. Generally you will only be able to get your home listed in local listing. However it is possible to pay a flat fee to be able to put your home onto an MLS, though not all brokers will allow this.
Often, hiring an agent to help you buy a home has no cost to you. The buying agent splits the commission with the selling agent, so essentially the seller is paying for your agent. Though when you are looking to buy a home that is being sold by the owner, you must pay your agent yourself. So for the luxury of having your home placed on an MLS, hiring a buying agent, which you may not even have to pay for, comes with many benefits.
Hiring a selling agent is more expensive than a buying agent, but is still often worth it. Many people who try to sell their homes without an agent end up having to lower their asking price in order to attract buyers. By having a selling agent, you will be less likely to have to lower your asking price, and the commission you will pay your agent could, in the end, save you money.