Ayup & welcome! We all started somewhere!

There are loads of theories, but as others have said, nobody don't knows - especially why it's the only one. (There have been claims that a cuppla others had at least one 'lintel'-type arrangement but as far as I know, the evidence was pretty non-existent).

There are loads of theories of what Stonehenge was for, and many of those involve the lintels. They range from the very plausible (for example, to create a lightbox arrangement for the sun and/or moon at the solstices) to the (to most people) completely ridiculous (I'm not getting into those!!!) and every point inbetween (for example, that it had a roof).

Have a google of it! ;^)

love

Moth