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Once in the tool, you can get estimates for a group of keywords and see how different budgets and bids will affect your position, CTR and search volume potential.
Keep in mind these are very rough estimates and will vary depending on how well written your ads are along with how well you group your keywords together (think tight themes as opposed to having too many semi-related keywords in each adgroup) and how relevant and navigable your landing pages are. Not to mention which geographical areas you choose to target.
Google even places this disclaimer when using the tool: "Because your campaigns do not yet have a performance history, keyword estimates are based on system-wide performance information."
So as you can see you'll at least be able to get the average performance based on the current bids and quality score. Run a tighter campaign and you can actually outperform the estimates given.