<p>I'm printing on an Ender 5 with the default flex/magnetic build surface.
I read that PLA and PETG may sometimes be printed without any bed heating at all and also that bed heating is the main contributor to the power consumption of a printer.</p>

<p>As I do see that bed heating definitely helps with the first layer adhesion I did not want to turn it off completely, but I did start experimenting with turning off bed heating after all solid bottom layers have printed (using the ChangeAtZ script in Cura) and so far I haven't seen any negative effects, especially no warping (I am usually printing with a brim or raft; I think that might also help in that regard).</p>

<p>Am I missing something? Why is <em>anyone</em> keeping the bed heated for an entire print?</p>

All the valid points of bed shrinking and dislocating your parts when cooling from the other answer aside there is also the added complexities both in testing reliability of such a thing that may or may not be applicable to all materials.
The added complexity to the slicers to figure out when it is safe to turn off the bed which I would imagine depends on part footprint. I also sometimes print several parts sequentially in the same job so then it would need to know that and time the bed heating correctly or pause and wait for bed between parts.
I would also categorize printer power use as trivial (order of three 60W lightbulbs), but considering millions of machines worldwide economics of scale does kick in.