I hope the fine author of Lingon doesn't disapprove of me using his app to answer this, but here are the reasons why helpd will run.
(the daemon of help)
It's hard to tell if this is one of the processes that needs to run a bit the first time it starts and then it will happily update itself more rapidly when needed (like /Applications being changed) as well as early every Sunday AM.
The virtual memory system should keep the RAM fine, so unless you see it constantly - it's just taking the RAM it needs to serve your help menu with fresh and up to date documents.
You can also disable that launchd job if it's causing you grief until you can isolate why.