You need to get a divorce if you wish to remarry. In a divorce, the marriage is dissolved and any legal ties between the parties are broken by law. Sometimes parties may wish to remain married so that they can continue to take advantage of benefit programs which only apply to married spouses. This can even help reduce support costs in some cases. In other situations, parties simply do not wish to go through with the hassle and costs involved in getting a divorce.
You do not need to get a divorce in order to be entitled to your share of the matrimonial property, child or spousal support or custody of a child. All of these issues can be dealt with outside of court through a separation agreement.
Conversely you can get a divorce without dealing with the other issues resulting from the breakdown of your relationship. However, it is important to note that after a divorce you have only two years to initiate court proceedings against an ex-partner for relief. When you do not get divorced you have six years from the date of your separation to initiate a similar proceeding.