All of the above are good points, but I'll suggest the real reason some companies have multiple sites. Let us take academic answers as an example (uk essays, law essays, oxbridge graduates, scanmyessay, markmyessay etc).
1. As above, marketting - each site links to all others. Someone uses scanmyessay for free and they may be tempted to click on the uk essays link.
2. (This is the most important in a business sense) Multiple companines = all monetary losses are 'balanced'. i.e. say uk essays makes 20,000 porfit, whilst law essays makes a 20,000 loss. By having two 'companies' the parent company academic answers ltd can claim a nil gain = no tax.
