New shares are usually issued after the start of the financial year. As a result, they are not fully entitled to dividend payments, and are listed and traded on the exchange separately from the "old" shares until dividends have been paid. Before being admitted to trading on the exchange, new shares are traded in the regulated unofficial market and off-floor trading.
Existing shareholders usually have subscription rights to new shares. By purchasing new shares, they can ensure that they own the same percentage of the company's capital stock as they did prior to the capital increase.