There are those who do not wish to rely upon electronic credit/debit transactions. Instead there are thise in favor of carrying and holding onto there cash. Some may still wish to accept cash only as well and I feel it's up to the individual. For cases where large monetary transactions may be needed, a large bill is a necessary option.
Higher denominations of currency mean less of lower denominations like 100s are needed. Since they are worth more money, the U.S. Treasury Department will need stringent anti-counterfeit measures just like they do on c-notes. Plus, when the new bills are recreated, collectors will be interest in the new currency. Just like the 50-state quarters program for coins, collectors will be intrigued by the new notes and pay even more for them in a secondary market.
I do not think that it would be beneficial to reprint the $500 and $1000 bill in the United States. It would possibly create problems for many cashiers that might experience these bills when someone has made a purchase. This could lead to possible fake money which is not a good idea at all.