Why do I need a delay as PIC powers up?
Did you experiment with the delay value, can it be lower?

More complex chips often have a power-up sequence during which they can't be used, like the (in)famous HD44780 LCD controller. I noticed this Tpor value in the nRF24L01+ datasheet (100 ms). I am not really sure what it specifies, but it could be interpreted as the time between the power reaching 1.8V and the chip being ready to handle commands.



Reading a bit further, the diagram on p22 confirms my interpretation.