Earlier quoted context omitted.
TSA pre-check is a pre-screening program that air travelers can use to bypass the normal security line. It's not a huge time-saver (any more, used to be more) but gets you out of a few annoyances like removing shoes, unpacking toiletries, and a few other things. And the lines are usually a bit shorter and the ratio of regular traveller to new traveller is a bit better (less likely to get caught behind somebody who do…
And they don't put you through the nudie scanner (or whatever it's called) which is important to some people. I have pre-check automatically through global entry (that basically lets you bypass immigration coming back into the US). Just got ETA for the UK and you also have e-entry (which usually works) from the US and a number of other countries as well.
And all the lines at IAD have the new, high-powered luggage scanners (for carry-on, so still less powerful than the ones used for checked bags). So for those of that fly with film camera gear, we need a hand check with or without pre-check.