To draw charts that range from minutes to several days. The data consists of a stream of single datalines (ticks), each with a bid/ask and eventually trade price/volume. Each time something happens (change in bid/ask or a trade), data is saved. So the more actively traded a stock is, the larger the datastream. Because this is a lot of data, it is often recalculated to certain intervals: 1 minute, 5 minutes, 15 minutes for charting purposes and for use in indicators.

To draw charts that range from days to years. Data consists of daily open, close, high, low and volume. Typical time intervals to draw the chart are weeks to years. The basic interval is 1 day (daily), but when you draw weekly or monthly charts, you actually recalculate the data to longer intervals, each with their own open, close, high, low and volume.

So after we've chosen the interval, the data is recalculated to intervals with open/close high/low and volume. On this interval data, all kinds of formulas can be applied to generate indicators, which we try to interpret as buy or sell signal.

This is very important to understand. Because indicators perform calculations on intervals, their value depends on the intervals we chose for them. This way, at a certain moment, an indicator could give a buy signal in a daily chart, while the same indicator would give a sell signal in a weekly chart. So we have to treat indicators with a lot of caution.