I do it on useless, "I know it's 5 days away" bids just to test if I'm signed in.
One time I was trying to snipe with 3 seconds left and a pop up window asked for my password, letting me know I wasn't signed in. I can still see my auctions, but not bid. I lost that auction nonetheless.
This way I put in a "throwaway bid", like when it's a few dollars for a genuine Rolex. If the bid goes through, then I know I'm logged in. I can then snipe on my actual bid with 2 seconds left. I know I'm not going to get that "throw away bid" anyways, so no worries about winning that Rolex. (although it would be nice to win on a $20 bid for one)