IF asking for advance payment doesn't prevent sales to close (which is why many companies avoid it), the benefits of getting advance payments are:

1) increased cash flow.
2) creating a deeper client commitment (making it more difficult to back out, stall, or change terms)
3) removes the burden on the vendor to go into "collections" mode which can be frustrating and difficult.
4) Gives the vendor a balance of leverage which can create a healthier relationship between both parties.
5) Gives the vendor the funds to proceed (assuming resources, etc. will be used). Otherwise, imagine resources are used to queue up the client, and then there is non-payment...the vendor then suffers a loss that may not be recovered.