Transferring files

Another novice! I just got a nuvi 350 and loaded some POI files to an SD card from this site with no problem. Now when I turn on the unit it asks me if I want to transfer the files from the SD card to internal memory. I have said "NO" mainly because I don't know if I can delete the data later if I choose. Is there an advantage to transferring the files and if I do how can I delete them later?
This is truly an amazing device with great sensitivity even indoors acquiring generally 9 of 12 satelites. I helped write the original government proposal for GPS in the late '70s. As I remember, at that time a man portable unit had to weigh less than 10 pounds, about the size of a laptop computer, and cost $3000-$5000. Of course they didn't have the dedicated chips then but the technology has sure come a long way.