This is a neat trick to improve the performance of your Sharepoint Box if you have custom webparts installed. Normally, the SharePoint indexer kicks in to index the pages, and when it does kick in, it causes an additional hit on the server. There may be cases, when you don't want your custom webparts to connect to backend data systems and process the data when the indexer is accessing it. So what we have done, is to check if the current account calling the page, is the service account or not. If it is the service account, then just dont do anything. This way, we bypass the entire processing of each of our custom webparts, when the indexer is accessing them and hence decrease the overall load on the server.
On many systems, the indexer is set to kick in every 10 - 20 mins, so it is quite a bit of saving on extremely busy systems.