If the item isn’t particularly rare or in high demand, then anybody else who comes by will say to themselves, “Why would I bid on an item that already has a bid on it when I can just bid on the other one nobody is paying attention to?”. Showing interest early on in an auction can make competitors pass on watching it altogether, instead of five people huddled around watching an item until the last seconds, not knowing if someone else will bid or not.
Hmmm, well there’s that.