Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
this could be useful when someone is trying to test a team that looks good but is unsure if the team will be actually good
this does not affect your ladder rating
If you want to play a couple games to test your team, you should try to go on the pokemon showdown room of the tier you're playing in, and then ask someone to do spare games with you. More often than not you'll find players here to help you.
I do think it would be valuable to have something like this. The problem with asking other players or using alts is that it's less likely to be matched up with someone of similar skill. I think some sort of alternate ladder that draws its matchmaking from the ranked ladder without affecting said ranked ladder could work. It doesn't necessarily need to flood the format selection menu. There could be a check box that toggles between the regular ladder and the frozen ladder.