In the show, the exact reason is never fully explained, but there are clear clues and popular interpretations. It’s a mix of “wrong place, wrong time” and the Upside Down needing him alive as a potential host or connection.
What the show makes clear
Will is not killed outright like Barb; instead he is dragged into the Upside Down and kept in a vine-like nest, with a slug implanted in him that later becomes a baby Demogorgon.
This shows the creatures sometimes keep humans alive as incubators or hosts rather than just food, which explains why Will was allowed to survive long enough to be rescued.
“Why Will?” in-story explanations
Several official and fan analyses point out that Will was alone, near a gate, and unlucky enough to be the one the Demogorgon encountered first, so part of it is simply that he crossed its path.
Later seasons show that the Mind Flayer and Vecna use human hosts, and Will becomes a key host in season 2, so many viewers infer that keeping him alive served a larger Upside Down plan rather than a random mercy.
Popular fan theories
Common theories say the Demogorgon (or its master) sensed something special about Will, like emotional vulnerability or a latent psychic sensitivity that made him useful as a connection to Eleven and the real world.
Others suggest that because the Upside Down hive-mind wanted a long-term link to Hawkins, Will was taken and preserved as a kind of “anchor” or chosen host instead of being eaten immediately.