Why do investors use year to date?
There's no natural break in the stock market. It's not a seasonal business, and the stock exchange is open for trading every weekday, with the exception of a few holidays.
Without a clear break in stock market activity, investors need to choose a starting point to measure the returns of a stock, and the first of the year offers a natural starting point since a new year naturally signals new beginnings.
Additionally, the end of the year generally coincides with a lull in trading activity and a relative lack of news. The week between Christmas and New Year's is traditionally a week when Americans, including finance professionals, take a vacation. The new year gives investors the chance to wipe the slate clean.
Because there's little news in the last week of the year, it also makes sense to start the year-to-date clock since there's unlikely to be any news that swings the stock substantially the way there is during, say, earnings season.