Keeping some extra ties on hand at work is smart for many reasons. For starters, what if you dip your tie in your spaghetti sauce at lunch? Or perhaps you dress down for work, but need to dress up for a business-related outing after work. And even if you've been wearing a tie all day, a new one can change the look of your whole ensemble for an evening event.
Also -- and let's hope this isn't the case -- if you pull a lot of all-nighters at the office, you'll have a different tie to wear the following day.