Pre-orders work really well for those who have a regular release schedule of 1-3 monthly, so you can always have the next book in the series on pre-order. They work well for a rapid-release strategy: book 1 goes live, with book 2 and possibly book 3 on pre-order. They work well for anyone who has a lengthy pre-release procedure of beta readers, ARCs, etc, so you put the book on pre-order while you do the final polish, knowing it will be done on time. They DON'T work well for anyone whose production schedule is erratic, or who likes to just finish the book and bang it up on Amazon.
For anyone who does want to try it, the procedure is exactly as per uploading a finished book except that you specify a date for release and nowadays you don't even have to upload a file at this stage. But you set keywords, categories, blurb and pricing, exactly the same. The lock-out period is (I think) 10 days [ETA: Vanessa's right - it's 4 days] but they give you loads of warning and there's an actual countdown on the KDP page so you know how long you've got to the second. You can release early, although I think it can't be instantaneous - you have to pick a date 3-4 days away. If you miss the lock-out deadline without good reason, they ban you from pre-orders for a year, but they're sympathetic to tales of fire, flood, tempest and a death in the family.
I love pre-orders. They work really well for me, but I have a steady production schedule (currently 3-monthly) so the next book in series is always on pre-order and readers can go straight from the link at the back of the book to pre-order the next, if they want. Then on release day, I get a tidy lump sum of sales money, while the KU reads are kicking in. Also, there's now some rank benefit on release day from accumulated pre-orders, so you get a rank boost when the pre-order happens AND some benefit on release day. Which is very nice. But pre-orders aren't for everyone, and if you've managed fine without them all this time, Tim, I wouldn't bother with them now.