The answer to this depends strongly on your company. However, in many large companies, there are so many people, so many proposed courses of action in any circumstance, and so many decisions to be made, that people rely on shortcuts. One of the most reliable is reputation. Someone might say "Well, if Kate is behind it I think we can be sure it will succeed. She's smart, she works well with others, she communicates well and the last three projects she was on all succeeded. I don't think she'd be involved with this if it was a bad idea." That's so much quicker for them than actually investigating my new project.
Your boss wants you to be the subject of a sentence like that in the near future. Partly as an altruistic gesture to help your career in the company. And partly as a selfish gesture to help other projects belonging to your boss. You've done well at something; let's make sure the whole company knows. You're smart enough to be on that committee; get on it and make sure everyone else on it knows you and sees what a winner you are. Some day soon, a decision will roll around that will go the way your boss wants, if only most of the room knows you and how good you are. This will help you and your boss. And yes, your appraisal, but this is a longer term play than this year's review.
If you work in a company where visibility matters, you are lucky to have a boss who knows that and will lead you through it. Do not reject that gift because you think you know how companies should work.
But on the other hand: there are occasionally little islands of "it's who you know" in a large company, who just schmooze and flatter and make sure they're visible without ever getting anything useful done, without being good or smart or wise. If you have such a boss and the rest of the company hates that, tread very very carefully. This is the rarer case but it can happen. Your best asset here is a friend in another organization of your company who can tell you which kind of situation you are in.
