If you have copper pipes with lead solder, or fixtures with brass (which will have varying amounts of lead), or lead supply lines from your city, the longer tha

If you have copper pipes with lead solder, or fixtures with brass (which will have varying amounts of lead), or lead supply lines from your city, the longer that water sits in contact with those joints/fixtures, the more lead it gets a chance to absorb. So flushing the line gets you 'fresher' water from the municipal line that hasn't dwelled against your smaller diameter dirtier pipe. Can be a big issue in schools for the first kids that drink from the fountains for the day. The water heaters, well, hotter water has higher solubility, so while it sits in pipes, could absorb more from the pipes.