Why keep the bed heated after initial layer(s) with PLA (or PETG)?

There are three reasons (I can think of):

1. A large problem you'd face with allowing the bed to cool after first layer is you stand the chance of losing adhesion after it cools. When you heat the bed, it expands somewhat. When it cools it contracts. It has been known for parts to actually pop off the bed if left on there to cool (after a print). If you allow the bed to cool fully, you could ruin a print due to it losing the adhesion, popping off the bed, then the printer keeps on going.

2. When you're dealing with 0.1 mm layer height, that's not a lot of wiggle room. When you level your bed before printing, it should be done after everything is heated. If you were to turn off the bed after you start printing, you could very easily shift the bed enough to take up the worth of an entire layer, which means your print has adjusted and will then have major imperfections. This isn't a given, but definitely a concern ... especially for larger or taller prints.

3. Whether PLA or PETG, the extruded filament needs to have heat in order to stay. This is not only heat in the extruder, but heat in the print itself. If the print cools off, this could affect subsequent adhesion for the filament getting laid down. If you turn the bed heater off after print start, you'll lose that heated environment. The print will cool off and you'll start seeing variations in the print, which, if the print is large enough, would most likely be more noticable. Think of it as a heated environment, not just putting piling host plastic on top of each other.

There may be other reasons, but I believe these are very good reasons not to turn your bed off after print start. If you are worried your power supply isn't providing enough power, then get a bigger power supply. If you're worried about power consumption overall, once the bed is heated, consumption goes way down (as @r_ahlskog stated in their answer).