Sites offer the web mass market the option to watch a live streaming movie at practically no cost. Their movie selections comprise those already available in the public domain to ensure no copyright infringements or privacy issues. It's probably just as well to offer a fair chance to viewers to either stay at home or jostle in cinemas for the added experience of giant screens and surround-sound.
Not to sell short on the experience of home viewing, viewers normally gravitate towards convenience. Perhaps your size of your home television screen and its sound system is nothing to compare with the screens and audio systems at your local cinemas, the thought of getting into your car, fighting through traffic and getting a parking space is perhaps more daunting. It's also a great excuse to have family and friends over for a meal and movie. Certainly a much cheaper option with added merriment as one can talk and make as much noise as possible without being shushed. The ability to suspend a live streaming movie for a restroom break or to soothe a fidgeting child is definitely a plus point.
Some movie sites also offer services for its clients or viewers to chat about their streamed movies. In encouraging an exchange of ideas and comments, everyone functions as an informal movie reviewer. Perhaps the official movie reviewers scan these sites to pick up the vibes on a latest release's acceptance by the mass market. A box office is more than just great ticket sales but also great reviews after.
If a title is not available in their catalog, some sites allow their clients or viewers to request for said movie. So long as there are no conflicting issues as well as existing presence in the public domain, the sites are more than happy to fulfill a request and add to their existing collection. As such, they value all input as more viewers equal more eyeballs for advertisement and marketing which are these sites' main source of revenue.