You make a move on a girl or ask her out.
She looks you firm in the eyes, aaaaand... "I thought we could just be friends!" she tells you.
Just like that, the wind has sucked all the way out of your sails.
I guess you have to just be friends with her now.
I'm kidding. What point is there to that?
You're talking to her because you want her. Either to date her or to have a fun roll in the hay with her.
When she puts the "let's just be friends" (LJBF) frame on you, you're faced with several options... but not all these options are created equal.