But why does showing work, where telling doesn't? I think it's because we ask the reader to do some of the work, and the more work the reader does, the more absorbed they are. Imagine being in the audience of a third rate tennis match. You watch the game, but your attention wanders...perhaps you could go for an icecream at the next break, ooh, that woman's wearing a funny dress, I wonder if it's going to rain...Then the cry goes up - the umpire has had to retire and you - yes, you - are going to take over. Now you have to watch each point carefully, make decisions and without noticing, the next hour flies past.
It's the same with writing. 'Imogen felt nervous' only requires the reader to absorb the information. It's passive, attention may wander, the book may be put down. Compare it to: 'I feel fine,' Imogen said, clearing her throat and wiping sweaty palms on her crumpled dress. Now the reader has to pay attention, has to deduce that while Imogen may claim to feel fine, her actions show us that she doesn't. The reader has to be active, has to work, has to pay attention. This reader will read to the end and find it a satisfying, worthwhile experience - and that's what we all want, isn't it?
PS And if you're still not convinced think of this. The first version takes 3 words. The other is 16. Just think of difference to your daily word count!