For example: The Terra Firma Coffee Company has 100 shares of stock outstanding. You own 10 of these shares, or 10% of the entire company. To raise capital to expand, the Board of Directors decides to sell another 100 shares in the company for $50 each. If the preemptive right did not exist, this would dilute your ownership to 5% (10 shares divided by 200 shares outstanding). You excercise your preemptive right to maintain your proportional interest and agree to buy (or "subscribe") to 10 shares of the new stock. You promptly cut a check for $500 (10 new shares x $50 offering price = $500) and now you own 20 shares out of 200 outstanding; the same 10%.
Some companies choose to do away with the preemptive right because it can be inconvenient when attempting to raise cash from equity issuance.

