Many day traders use technical analysis and charts and would recommend a 'clean' approach to trading strategy. These traders prefer not to load their charts with lots of different indicators in order to try and second-guess direction. Rather, they will focus solely on price; this is often referred to as 'price-action trading'. There is definitely some merit to this and, when trading in this way, you still have some key reference points based on what has happened previously to help you plan future trades.
For some day traders the previous day’s high and low are important levels to watch when it comes to planning a strategy for today. This is actually quite logical: yesterday's high marked the point where sentiment changed and the sellers came back into the market and pushed the price lower. The market consensus was that the price was too high. And of course the previous day's low shows where the buyers regained confidence as they felt the market was undervalued – they voted with their wallets and bought. It is not too much of a stretch of the imagination to think that these levels could well be important if they come into play again, and this can be the cornerstone of a day trading strategy.