As far as I think, sponsoring is just not required. If you are coming to the US, you can survive by working in McDonalds. Unless you have less than 5 thousand dollars per person in the Nepalese bank account, sponsorship is not required. However, due to monetary regulations, you can only bring certain amount of money here. So, for that purpose, having a welcome letter will help because that way, you are not initially in a burden to pay rent.