The
page file is used by the OS primarily when it runs out of RAM - it puts some little used info from RAM onto disk to free up space for something more important. By default windows has a page file - so yours has been turned off by someone or something.
The problem is, people think 'I've got plenty RAM so don't need a page file' but this is not necessarily true. Some programs, particularly ones that were written at times when memory limits were tighter, would write information directly to the page file if they knew this information would be little used. Other programs would have checks to ensure there was a certain amount of page file free in case they needed it, and so on. As a result, turning it off isn't a good idea.
On why people turn it off: They think that they've got enough memory for everything they want to run (possibly true) and that windows will move stuff they want to the page file rather than keep it in memory (unlikely if the first point was true) and that this will result in poor performance (unlikely as it'll move little-used stuff there first and has tons of logic to try and do this well). In practise in most scenarios this either has no effect or makes performance worse as instead of moving stuff to page file it'll change behaviour to reduce memory use, so it'll pre-fetch less content for stuff you actually are using, resulting in a worse experience in most cases (not all - some benchmarks will be faster the other way round)