Comment by pmontra

That's done by arranging a demo (the very old way) or (better) by deploying to a staging server. The customer meets with you for a demo not very often, maybe once per month, or checks what's on the staging server maybe a couple of times per week. They have other things to do, so you cannot make them check your proposal multiple times per day. However I concede that if you are fast you can work for multiple customers at the same time and juggle their demos on the staging servers.